From 679ed8936f023df39fc6bcaf33e88f158968a768 Mon Sep 17 00:00:00 2001
From: Matthew Batchelder <borkweb@gmail.com>
Date: Sun, 16 Aug 2026 10:03:39 -0700
Subject: [PATCH 1/4] test(php-transformer): freeze auto-fit grid and css-owned
 flex contracts
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

## Summary
Adds two deliberately RED contracts, one per confirmed defect: `repeat(auto-fit, …)` track lists must not be converted to a native WordPress grid layout attribute, and a flex container demoted to a css-owned core/group must keep its authored `display:flex`.

## Why
`StyleResolutionTrait::autoRepeatMinimumColumnWidth()` accepts both `auto-fit` and `auto-fill` and converts both to `layout.minimumColumnWidth`. `wp-includes/block-supports/layout.php` hardcodes `auto-fill` in every branch that renders that attribute, so an authored `auto-fit` grid renders with empty tracks retained and the content crammed into part of the measure.

`cssOwnedGroupAttributes()` drops the native `layout` attribute on demotion. Grids survive because `CSS_OWNED_GRID_CARRIER_PROPERTIES` rides their inline declarations to the generated stylesheet; there is no flex equivalent, so the block is marked `blocks-engine-css-owned-flow` while the declaration that makes it a flex container is dropped and the children stack.

## How
`tests/unit/auto-fit-grid-carrier.php` covers inline and class-owned `auto-fit` track lists plus `auto-fill` controls proving the native conversion is retained, not abandoned. `tests/unit/css-owned-flex-carrier.php` covers inline row and column flex containers plus controls for a container with no authored display and for a class-owned flex declaration the author stylesheet already owns.

## Testing
RED evidence at this commit: `php tests/unit/auto-fit-grid-carrier.php` exits 1 with 6 failed and 5 passed assertions; `php tests/unit/css-owned-flex-carrier.php` exits 1 with 6 failed and 6 passed assertions.
---
 php-transformer/composer.json                 |   2 +
 .../tests/unit/auto-fit-grid-carrier.php      | 141 ++++++++++++++++
 .../tests/unit/css-owned-flex-carrier.php     | 154 ++++++++++++++++++
 3 files changed, 297 insertions(+)
 create mode 100644 php-transformer/tests/unit/auto-fit-grid-carrier.php
 create mode 100644 php-transformer/tests/unit/css-owned-flex-carrier.php

diff --git a/php-transformer/composer.json b/php-transformer/composer.json
index 7d7a140e..5a8a61a0 100644
--- a/php-transformer/composer.json
+++ b/php-transformer/composer.json
@@ -80,6 +80,8 @@
       "php tests/unit/engine-support-css-asset.php",
       "php tests/unit/engine-support-css-specificity.php",
       "php tests/unit/inline-display-carrier.php",
+      "php tests/unit/css-owned-flex-carrier.php",
+      "php tests/unit/auto-fit-grid-carrier.php",
       "php tests/unit/artifact-author-stylesheet-projection.php",
       "php tests/unit/fallback-finding-normalizer.php",
       "php tests/unit/navigation-underline-color-resolver.php",
diff --git a/php-transformer/tests/unit/auto-fit-grid-carrier.php b/php-transformer/tests/unit/auto-fit-grid-carrier.php
new file mode 100644
index 00000000..14641c4e
--- /dev/null
+++ b/php-transformer/tests/unit/auto-fit-grid-carrier.php
@@ -0,0 +1,141 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Contract for `repeat(auto-fit, …)` track lists.
+ *
+ * `wp-includes/block-supports/layout.php` hardcodes `auto-fill` in every branch
+ * that renders `minimumColumnWidth`, so a native grid layout attribute can only
+ * ever express `auto-fill`. Converting an authored `auto-fit` track list to that
+ * attribute changes the rendered geometry: `auto-fit` collapses empty tracks,
+ * `auto-fill` retains them, so the content is crammed into part of the measure.
+ * `auto-fit` must therefore stay under CSS ownership like every other track list
+ * WordPress cannot express, while `auto-fill` keeps its native conversion.
+ */
+
+require dirname(__DIR__, 2) . '/vendor/autoload.php';
+
+use Automattic\BlocksEngine\PhpTransformer\HtmlToBlocks\HtmlTransformer;
+
+$failures = 0;
+$passes = 0;
+
+$assert = static function (bool $condition, string $message, string $detail = '') use (&$failures, &$passes): void {
+    if ( $condition ) {
+        ++$passes;
+        return;
+    }
+
+    ++$failures;
+    fwrite(STDERR, 'FAIL: ' . $message . ('' !== $detail ? ' - ' . $detail : '') . PHP_EOL);
+};
+
+$cssFor = static function (array $result, string $source): string {
+    return implode("\n", array_map(
+        static fn (array $asset): string => (string) ($asset['content'] ?? ''),
+        array_values(array_filter(
+            is_array($result['assets'] ?? null) ? $result['assets'] : array(),
+            static fn (array $asset): bool => $source === ($asset['source'] ?? '')
+        ))
+    ));
+};
+
+$transform = static fn (string $html): array => ( new HtmlTransformer() )->transform($html, array())->toArray();
+
+$cards = '<div><p>One</p></div><div><p>Two</p></div><div><p>Three</p></div>';
+
+// -- Defect: an inline auto-fit track list is converted to a native grid layout.
+$inlineAutoFit = $transform(
+    '<section class="cards" style="display:grid;grid-template-columns:repeat(auto-fit, minmax(280px, 1fr));gap:24px">' . $cards . '</section>'
+);
+$inlineAutoFitMarkup = (string) ($inlineAutoFit['serialized_blocks'] ?? '');
+$inlineAutoFitEngineCss = $cssFor($inlineAutoFit, 'engine-support');
+
+$assert(
+    ! str_contains($inlineAutoFitMarkup, 'minimumColumnWidth'),
+    'inline auto-fit: no native grid layout attribute, which WordPress would render as auto-fill',
+    $inlineAutoFitMarkup
+);
+$assert(
+    str_contains($inlineAutoFitMarkup, 'blocks-engine-css-owned-grid'),
+    'inline auto-fit: container is marked css-owned-grid like every other non-expressible track list',
+    $inlineAutoFitMarkup
+);
+$assert(
+    str_contains($inlineAutoFitEngineCss, 'grid-template-columns:repeat(auto-fit, minmax(280px, 1fr))'),
+    'inline auto-fit: the authored track list rides to the generated stylesheet verbatim',
+    $inlineAutoFitEngineCss
+);
+$assert(
+    str_contains($inlineAutoFitEngineCss, 'display:grid'),
+    'inline auto-fit: the carrier keeps the container a grid',
+    $inlineAutoFitEngineCss
+);
+
+// -- Same defect through a class-owned track list, which is the shape the
+// site-builder corpus authors. The author stylesheet already retains the rule;
+// the native attribute is what overrides it.
+$classAutoFit = $transform(
+    '<style>.cards{display:grid;grid-template-columns:repeat(auto-fit, minmax(280px, 1fr));gap:24px}</style>'
+    . '<section class="cards">' . $cards . '</section>'
+);
+$classAutoFitMarkup = (string) ($classAutoFit['serialized_blocks'] ?? '');
+$classAutoFitAuthorCss = $cssFor($classAutoFit, 'author-css');
+
+$assert(
+    ! str_contains($classAutoFitMarkup, 'minimumColumnWidth'),
+    'class-owned auto-fit: no native grid layout attribute',
+    $classAutoFitMarkup
+);
+$assert(
+    ! str_contains($classAutoFitMarkup, 'is-layout-grid'),
+    'class-owned auto-fit: no core grid layout classes competing with the author rule',
+    $classAutoFitMarkup
+);
+$assert(
+    str_contains($classAutoFitAuthorCss, '.cards{display:grid;grid-template-columns:repeat(auto-fit, minmax(280px, 1fr));gap:24px}'),
+    'class-owned auto-fit: the author rule stays the single owner of the track geometry',
+    $classAutoFitAuthorCss
+);
+
+// -- Control: auto-fill IS natively expressible and must keep converting.
+$inlineAutoFill = $transform(
+    '<section class="cards" style="display:grid;grid-template-columns:repeat(auto-fill, minmax(280px, 1fr));gap:24px">' . $cards . '</section>'
+);
+$inlineAutoFillMarkup = (string) ($inlineAutoFill['serialized_blocks'] ?? '');
+$inlineAutoFillEngineCss = $cssFor($inlineAutoFill, 'engine-support');
+
+$assert(
+    str_contains($inlineAutoFillMarkup, '"layout":{"type":"grid","minimumColumnWidth":"280px"}'),
+    'auto-fill control: still converts to the native grid layout attribute',
+    $inlineAutoFillMarkup
+);
+$assert(
+    ! str_contains($inlineAutoFillMarkup, 'blocks-engine-css-owned-grid'),
+    'auto-fill control: does not fall back to the css-owned-grid carrier',
+    $inlineAutoFillMarkup
+);
+$assert(
+    ! str_contains($inlineAutoFillEngineCss, 'grid-template-columns'),
+    'auto-fill control: WordPress still owns the track geometry',
+    $inlineAutoFillEngineCss
+);
+
+$classAutoFill = $transform(
+    '<style>.cards{display:grid;grid-template-columns:repeat(auto-fill, minmax(280px, 1fr));gap:24px}</style>'
+    . '<section class="cards">' . $cards . '</section>'
+);
+$classAutoFillMarkup = (string) ($classAutoFill['serialized_blocks'] ?? '');
+
+$assert(
+    str_contains($classAutoFillMarkup, '"layout":{"type":"grid","minimumColumnWidth":"280px"}'),
+    'class-owned auto-fill control: still converts to the native grid layout attribute',
+    $classAutoFillMarkup
+);
+
+if ( $failures > 0 ) {
+    fwrite(STDERR, "Auto-fit grid carrier contract: {$failures} failed, {$passes} passed\n");
+    exit(1);
+}
+
+fwrite(STDOUT, "Auto-fit grid carrier contract passed: {$passes} assertions\n");
diff --git a/php-transformer/tests/unit/css-owned-flex-carrier.php b/php-transformer/tests/unit/css-owned-flex-carrier.php
new file mode 100644
index 00000000..bfb33fb7
--- /dev/null
+++ b/php-transformer/tests/unit/css-owned-flex-carrier.php
@@ -0,0 +1,154 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Contract for inline `display:flex` on containers demoted to CSS-owned layout.
+ *
+ * `cssOwnedGroupAttributes()` drops the native `layout` attribute when an author
+ * flex/grid container is demoted to a css-owned core/group. Grids survive that
+ * demotion because their inline declarations ride to the generated stylesheet on
+ * a carrier class. Flex containers had no equivalent, so the declaration that
+ * makes the container a flex container was dropped while the block was still
+ * marked `blocks-engine-css-owned-flow` — the children then stacked.
+ */
+
+require dirname(__DIR__, 2) . '/vendor/autoload.php';
+
+use Automattic\BlocksEngine\PhpTransformer\HtmlToBlocks\HtmlTransformer;
+
+$failures = 0;
+$passes = 0;
+
+$assert = static function (bool $condition, string $message, string $detail = '') use (&$failures, &$passes): void {
+    if ( $condition ) {
+        ++$passes;
+        return;
+    }
+
+    ++$failures;
+    fwrite(STDERR, 'FAIL: ' . $message . ('' !== $detail ? ' - ' . $detail : '') . PHP_EOL);
+};
+
+$cssFor = static function (array $result, string $source): string {
+    return implode("\n", array_map(
+        static fn (array $asset): string => (string) ($asset['content'] ?? ''),
+        array_values(array_filter(
+            is_array($result['assets'] ?? null) ? $result['assets'] : array(),
+            static fn (array $asset): bool => $source === ($asset['source'] ?? '')
+        ))
+    ));
+};
+
+$transform = static fn (string $html): array => ( new HtmlTransformer() )->transform($html, array())->toArray();
+
+/** Rule body for the geometry carrier class present in the markup. */
+$carrierRule = static function (string $markup, string $css): string {
+    if ( 1 !== preg_match('/\b(be-inline-geometry-[a-f0-9-]+)\b/', $markup, $match) ) {
+        return '';
+    }
+    if ( 1 !== preg_match('/(?<![\w-])\.' . preg_quote($match[1], '/') . '\{([^}]*)\}/', $css, $rule) ) {
+        return '';
+    }
+
+    return (string) $rule[1];
+};
+
+$columns = '<div><p>Alpha</p></div><div><p>Beta</p></div><div><p>Gamma</p></div>';
+
+// -- Defect: a css-owned-flow footer loses its inline display:flex entirely.
+$flex = $transform(
+    '<footer style="display:flex;gap:2rem;justify-content:space-between;align-items:center">' . $columns . '</footer>'
+);
+$flexMarkup = (string) ($flex['serialized_blocks'] ?? '');
+$flexEngineCss = $cssFor($flex, 'engine-support');
+$flexRule = $carrierRule($flexMarkup, $flexEngineCss);
+
+$assert(
+    str_contains($flexMarkup, 'blocks-engine-css-owned-flow'),
+    'inline flex: the container is still demoted to a css-owned-flow group',
+    $flexMarkup
+);
+$assert(
+    1 === preg_match('/\bbe-inline-geometry-[a-f0-9-]+\b/', $flexMarkup),
+    'inline flex: the demoted container receives a geometry carrier class',
+    $flexMarkup
+);
+$assert(
+    str_contains($flexRule, 'display:flex'),
+    'inline flex: the carrier keeps the container a flex container',
+    '' !== $flexRule ? $flexRule : $flexEngineCss
+);
+$assert(
+    str_contains($flexRule, 'gap:2rem'),
+    'inline flex: the authored gap rides with the display declaration',
+    '' !== $flexRule ? $flexRule : $flexEngineCss
+);
+$assert(
+    str_contains($flexRule, 'justify-content:space-between'),
+    'inline flex: the authored main-axis distribution rides with the display declaration',
+    '' !== $flexRule ? $flexRule : $flexEngineCss
+);
+$assert(
+    str_contains($flexRule, 'align-items:center'),
+    'inline flex: the authored cross-axis alignment rides with the display declaration',
+    '' !== $flexRule ? $flexRule : $flexEngineCss
+);
+
+// -- A vertical flex column keeps its direction rather than silently becoming a row.
+$column = $transform(
+    '<footer style="display:flex;flex-direction:column;flex-wrap:wrap;gap:1rem">' . $columns . '</footer>'
+);
+$columnMarkup = (string) ($column['serialized_blocks'] ?? '');
+$columnRule = $carrierRule($columnMarkup, $cssFor($column, 'engine-support'));
+
+$assert(
+    str_contains($columnRule, 'display:flex') && str_contains($columnRule, 'flex-direction:column') && str_contains($columnRule, 'flex-wrap:wrap'),
+    'inline flex column: direction and wrap ride with the display declaration',
+    '' !== $columnRule ? $columnRule : $cssFor($column, 'engine-support')
+);
+
+// -- Control: no authored display gains no carrier and no flex declaration.
+$plain = $transform('<footer class="plain">' . $columns . '</footer>');
+$plainMarkup = (string) ($plain['serialized_blocks'] ?? '');
+$plainEngineCss = $cssFor($plain, 'engine-support');
+
+$assert(
+    1 !== preg_match('/\bbe-inline-geometry-[a-f0-9-]+\b/', $plainMarkup),
+    'no authored display control: no geometry carrier class is invented',
+    $plainMarkup
+);
+$assert(
+    ! str_contains($plainEngineCss, 'display:flex'),
+    'no authored display control: no flex declaration is invented',
+    $plainEngineCss
+);
+$assert(
+    str_starts_with($plainMarkup, '<!-- wp:group {"className":"plain","tagName":"footer"} --><footer class="wp-block-group plain">'),
+    'no authored display control: the container block is unchanged',
+    substr($plainMarkup, 0, 200)
+);
+
+// -- Control: a class-owned flex container has nothing inline to carry, and the
+// author stylesheet already retains the declaration.
+$classFlex = $transform(
+    '<style>.bar{display:flex;gap:2rem}</style><footer class="bar">' . $columns . '</footer>'
+);
+$classFlexMarkup = (string) ($classFlex['serialized_blocks'] ?? '');
+
+$assert(
+    1 !== preg_match('/\bbe-inline-geometry-[a-f0-9-]+\b/', $classFlexMarkup),
+    'class-owned flex control: no carrier is generated for declarations the author stylesheet owns',
+    $classFlexMarkup
+);
+$assert(
+    str_contains($cssFor($classFlex, 'author-css'), '.bar{display:flex;gap:2rem}'),
+    'class-owned flex control: the author rule stays the owner of the flex declaration',
+    $cssFor($classFlex, 'author-css')
+);
+
+if ( $failures > 0 ) {
+    fwrite(STDERR, "CSS-owned flex carrier contract: {$failures} failed, {$passes} passed\n");
+    exit(1);
+}
+
+fwrite(STDOUT, "CSS-owned flex carrier contract passed: {$passes} assertions\n");

From e43809829a3b904985802e751fc9c0e8d88de92c Mon Sep 17 00:00:00 2001
From: Matthew Batchelder <borkweb@gmail.com>
Date: Sun, 16 Aug 2026 10:27:39 -0700
Subject: [PATCH 2/4] fix(php-transformer): keep auto-fit grids and demoted
 flex containers intact
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Restricts the native grid-layout conversion to `auto-fill`, and carries the inline flex declarations of a container demoted to a css-owned core/group.

`autoRepeatMinimumColumnWidth()` accepted `auto-fit` and `auto-fill` alike and converted both to `layout.minimumColumnWidth`. `wp-includes/block-supports/layout.php` hardcodes `auto-fill` in every branch that renders that attribute, so an authored `auto-fit` grid rendered with the empty tracks `auto-fit` collapses still retained, squeezing the content into part of the measure. Measured against a design, delivered tracks came back `267 267 267 267` where the design resolved `592 592 0 0`.

`cssOwnedGroupAttributes()` drops the native `layout` attribute on demotion. Grids survive because `CSS_OWNED_GRID_CARRIER_PROPERTIES` rides their inline declarations to the generated stylesheet. There was no flex equivalent, so a container marked `blocks-engine-css-owned-flow` lost the declaration that made it a flex container and its children stacked.

`auto-fit` now returns '' from the conversion and falls through to the same css-owned-grid carrier that already holds every other track list WordPress cannot express, so the author rule stays the single owner of the exact authored track list. `auto-fill` keeps converting natively.

`CSS_OWNED_FLEX_CARRIER_PROPERTIES` plus `isCssOwnedFlexElement()`/`cssOwnedFlexAttributes()` mirror the grid carrier for flex containers. An inline display that overrides class-owned layout is skipped: #883 already carries those properties complete, at the non-important specificity tier, and forcing them would move them to the important tier.

- [ ] `composer test:unit` — 0 failures across 37 scripts, including the two new contracts at 11 and 12 assertions.
- [ ] `composer test` — 0 failures, 275 parity fixtures.

Seven parity fixtures moved, each because it encoded one of the two defects: three auto-fit fixtures asserted the native conversion (one renamed, since `html-autofit-grid-maps-to-minimum-column-width` named the buggy contract), and four flex fixtures asserted output in which the authored `display:flex` had been dropped.
---
 .../src/HtmlToBlocks/HtmlTransformer.php      | 62 +++++++++++++++++++
 .../Style/StyleResolutionTrait.php            | 17 +++--
 ...tofit-grid-carries-gap-and-background.json | 13 ++--
 .../html-autofit-grid-inline-leaf-items.json  | 11 ++--
 ...fit-grid-maps-to-minimum-column-width.json | 34 ----------
 .../html-autofit-grid-stays-css-owned.json    | 36 +++++++++++
 .../html-horizontal-flex-stays-columns.json   |  8 ++-
 .../html-single-child-flex-svg-address.json   |  9 ++-
 .../parity/html-single-child-flex-svg.json    |  8 ++-
 ...ml-vertical-flex-column-becomes-group.json |  8 ++-
 .../unit/block-style-support-conversion.php   | 13 +++-
 11 files changed, 157 insertions(+), 62 deletions(-)
 delete mode 100644 php-transformer/tests/fixtures/parity/html-autofit-grid-maps-to-minimum-column-width.json
 create mode 100644 php-transformer/tests/fixtures/parity/html-autofit-grid-stays-css-owned.json

diff --git a/php-transformer/src/HtmlToBlocks/HtmlTransformer.php b/php-transformer/src/HtmlToBlocks/HtmlTransformer.php
index 10a0b185..a1c2f86d 100644
--- a/php-transformer/src/HtmlToBlocks/HtmlTransformer.php
+++ b/php-transformer/src/HtmlToBlocks/HtmlTransformer.php
@@ -453,6 +453,21 @@ final class HtmlTransformer
         'place-items',
     );
 
+    /** @var list<string> Inline flex declarations carried to the generated stylesheet for css-owned flex containers. */
+    private const CSS_OWNED_FLEX_CARRIER_PROPERTIES = array(
+        'display',
+        'flex-flow',
+        'flex-direction',
+        'flex-wrap',
+        'gap',
+        'row-gap',
+        'column-gap',
+        'align-content',
+        'align-items',
+        'justify-content',
+        'place-content',
+    );
+
     private const CSS_OWNED_LAYOUT_ITEM_CLASS = 'blocks-engine-css-owned-layout-item';
 
     /** @var array<string, string> Source control DOM paths mapped to core/button wrapper classes. */
@@ -4403,6 +4418,10 @@ private function cssOwnedGroupAttributes(DOMElement $element): array
             return $this->cssOwnedGridAttributes($element);
         }
 
+        if ( $this->isCssOwnedFlexElement($element) ) {
+            $attrs = $this->cssOwnedFlexAttributes($element);
+        }
+
         unset($attrs['layout']);
         $attrs['className'] = $this->mergeClassNames(
             (string) ($attrs['className'] ?? ''),
@@ -4423,6 +4442,49 @@ private function cssOwnedGroupAttributes(DOMElement $element): array
         return $attrs;
     }
 
+    private function isCssOwnedFlexElement(DOMElement $element): bool
+    {
+        $display = strtolower(trim((string) preg_replace(
+            '/\s*!important\s*$/i',
+            '',
+            (string) ($this->structuralPresentationDeclarations($element)['display'] ?? '')
+        )));
+
+        return in_array($display, array( 'flex', 'inline-flex' ), true);
+    }
+
+    /**
+     * Attributes for a block hosting an author flex container demoted to CSS
+     * ownership. The demotion below drops the native `layout` attribute, which
+     * was the only thing expressing the flex container, so without carrying the
+     * authored `display:flex` the children stack. The inline declarations ride
+     * to the generated stylesheet on a carrier class exactly as
+     * CSS_OWNED_GRID_CARRIER_PROPERTIES does for grids; class-owned ones are
+     * already retained by author stylesheet materialization.
+     *
+     * An inline display that overrides class-owned layout is already carried
+     * complete by inlineGeometryClassName(), at the non-important specificity
+     * tier that keeps authored !important rules winning. Forcing those same
+     * properties would move them to the !important tier, so that case is left
+     * alone.
+     *
+     * @return array<string, mixed>
+     */
+    private function cssOwnedFlexAttributes(DOMElement $element): array
+    {
+        $inlineDeclarations = $this->cssDeclarations($this->attr($element, 'style'));
+        if ( $this->inlineDisplayOverridesAuthorLayout($element, $inlineDeclarations) ) {
+            return $this->presentationAttributes($element);
+        }
+
+        // Carry only the inline-present properties so the fallback to
+        // mapper-synthesized declarations cannot invent a `gap` that
+        // overrides explicit row-gap/column-gap values.
+        $carriedProperties = array_values(array_intersect(self::CSS_OWNED_FLEX_CARRIER_PROPERTIES, array_keys($inlineDeclarations)));
+
+        return $this->presentationAttributes($element, array(), $carriedProperties);
+    }
+
     private function isCssOwnedGridElement(DOMElement $element): bool
     {
         $display = strtolower(trim((string) preg_replace(
diff --git a/php-transformer/src/HtmlToBlocks/Style/StyleResolutionTrait.php b/php-transformer/src/HtmlToBlocks/Style/StyleResolutionTrait.php
index a9cd2e29..3b67768c 100644
--- a/php-transformer/src/HtmlToBlocks/Style/StyleResolutionTrait.php
+++ b/php-transformer/src/HtmlToBlocks/Style/StyleResolutionTrait.php
@@ -1993,15 +1993,22 @@ private function layoutFlexWrap(string $value): string
     }
 
     /**
-     * A track list of exactly repeat(auto-fit|auto-fill, minmax(<width>, 1fr))
-     * is natively expressible as WordPress grid layout: core renders
+     * A track list of exactly repeat(auto-fill, minmax(<width>, 1fr)) is
+     * natively expressible as WordPress grid layout: core renders
      * minimumColumnWidth as repeat(auto-fill, minmax(min(<width>, 100%), 1fr)).
-     * Every other track list (fixed counts, asymmetric tracks, nested
-     * functions) returns '' and stays under author CSS ownership.
+     *
+     * auto-fit is deliberately excluded. wp-includes/block-supports/layout.php
+     * hardcodes auto-fill in every branch that renders minimumColumnWidth, so
+     * the attribute cannot express auto-fit at all. The two keywords differ in
+     * rendered geometry — auto-fit collapses tracks left empty, auto-fill
+     * retains them — so converting auto-fit would keep the empty tracks and
+     * squeeze the real content into part of the measure. Like every other track
+     * list WordPress cannot express (fixed counts, asymmetric tracks, nested
+     * functions), auto-fit returns '' and stays under author CSS ownership.
      */
     private function autoRepeatMinimumColumnWidth(string $tracks): string
     {
-        if ( 1 === preg_match('/^repeat\(\s*auto-(?:fit|fill)\s*,\s*minmax\(\s*([0-9]*\.?[0-9]+(?:px|rem|em|ch|ex|vw|vh|vmin|vmax|%))\s*,\s*1fr\s*\)\s*\)$/i', trim($tracks), $matches)
+        if ( 1 === preg_match('/^repeat\(\s*auto-fill\s*,\s*minmax\(\s*([0-9]*\.?[0-9]+(?:px|rem|em|ch|ex|vw|vh|vmin|vmax|%))\s*,\s*1fr\s*\)\s*\)$/i', trim($tracks), $matches)
             && 0.0 < (float) $matches[1]
         ) {
             return strtolower($matches[1]);
diff --git a/php-transformer/tests/fixtures/parity/html-autofit-grid-carries-gap-and-background.json b/php-transformer/tests/fixtures/parity/html-autofit-grid-carries-gap-and-background.json
index 13649d1e..bbc3acce 100644
--- a/php-transformer/tests/fixtures/parity/html-autofit-grid-carries-gap-and-background.json
+++ b/php-transformer/tests/fixtures/parity/html-autofit-grid-carries-gap-and-background.json
@@ -1,11 +1,11 @@
 {
   "schema": "blocks-engine/php-transformer/parity-fixture/v1",
   "name": "html-autofit-grid-carries-gap-and-background",
-  "description": "An auto-fit grid using the hairline-divider technique (gap:1px plus a container background painting through the gaps) keeps its native grid tracks while source CSS owns its gap: core/group save markup cannot serialize blockGap. The container background remains a color support so the dividers survive without the author stylesheet.",
+  "description": "An auto-fit grid using the hairline-divider technique (gap:1px plus a container background painting through the gaps) stays entirely under author CSS ownership: core's grid layout support hardcodes auto-fill, which would retain the tracks auto-fit collapses. The author rule keeps the tracks, the hairline gap, and the divider background together, so the technique survives intact rather than being split across a native layout attribute and a color support.",
   "source_reference": {
     "repo": "php-transformer",
     "path": "tests/fixtures/parity/html-autofit-grid-carries-gap-and-background.json",
-    "notes": "Derived from a portfolio work grid where gap:1px;background:var(--ink) painted hairline separators between cells. The source stylesheet retains the gap because core/group does not serialize blockGap in canonical save markup."
+    "notes": "Derived from a portfolio work grid where gap:1px;background:var(--ink) painted hairline separators between cells. The gap and the background must stay with the track list in one author rule: promoting either to a block support while the tracks stay in CSS splits a single visual technique across two owners."
   },
   "legacy_comparison": {
     "skip": true,
@@ -20,7 +20,8 @@
       "path": "blocks.0",
       "name": "core/group",
       "attrs": {
-        "layout": { "type": "grid", "minimumColumnWidth": "240px" }
+        "layout": null,
+        "className": "work-grid blocks-engine-css-owned-layout blocks-engine-css-owned-grid"
       }
     }
   ],
@@ -29,7 +30,9 @@
     { "path": "status", "assert": "equals", "value": "success" },
     { "path": "fallbacks", "assert": "count", "count": 0 },
     { "path": "blocks.0.attrs.style.spacing.blockGap", "assert": "equals", "value": null },
-    { "path": "blocks.0.attrs.style.color.background", "assert": "equals", "value": "#1a1a1a" },
-    { "path": "serialized_blocks", "assert": "contains", "value": "is-layout-grid" }
+    { "path": "blocks.0.attrs.style.color.background", "assert": "equals", "value": null },
+    { "path": "serialized_blocks", "assert": "not_contains", "value": "is-layout-grid" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "blocks-engine-css-owned-grid" },
+    { "path": "assets.1.content", "assert": "contains", "value": ".work-grid{display:grid;grid-template-columns:repeat(auto-fit, minmax(240px, 1fr));gap:1px;background:#1a1a1a}" }
   ]
 }
diff --git a/php-transformer/tests/fixtures/parity/html-autofit-grid-inline-leaf-items.json b/php-transformer/tests/fixtures/parity/html-autofit-grid-inline-leaf-items.json
index cce091c7..88d766e6 100644
--- a/php-transformer/tests/fixtures/parity/html-autofit-grid-inline-leaf-items.json
+++ b/php-transformer/tests/fixtures/parity/html-autofit-grid-inline-leaf-items.json
@@ -1,11 +1,11 @@
 {
   "schema": "blocks-engine/php-transformer/parity-fixture/v1",
   "name": "html-autofit-grid-inline-leaf-items",
-  "description": "An expressible auto-fit grid whose direct children are standalone inline text leaves keeps native grid layout while source CSS owns its gap. Each leaf rides its own display:contents carrier paragraph, so the item count matches the source children and the spans themselves become the grid items. Locks the standalone-inline-leaf routing branch, which bypasses cssOwnedGroupAttributes.",
+  "description": "An auto-fit grid whose direct children are standalone inline text leaves keeps the author rule as the owner of its tracks and gap, because core's grid layout support cannot express auto-fit. Each leaf still rides its own display:contents carrier paragraph, so the item count matches the source children and the spans themselves become the grid items. Locks the standalone-inline-leaf routing branch, which bypasses cssOwnedGroupAttributes.",
   "source_reference": {
     "repo": "php-transformer",
     "path": "tests/fixtures/parity/html-autofit-grid-inline-leaf-items.json",
-    "notes": "Derived from client-logo strips built as bare styled spans inside repeat(auto-fit, minmax(W, 1fr)) grids. Guards against two failure modes: adjacent inline leaves coalescing into one paragraph (one grid item instead of N) and the branch dropping the native layout back to a vertical stack."
+    "notes": "Derived from client-logo strips built as bare styled spans inside repeat(auto-fit, minmax(W, 1fr)) grids. Guards against two failure modes: adjacent inline leaves coalescing into one paragraph (one grid item instead of N) and the retained author rule losing its display:grid, which would stack the strip vertically."
   },
   "legacy_comparison": {
     "skip": true,
@@ -19,7 +19,7 @@
     {
       "path": "blocks.0",
       "name": "core/group",
-      "attrs": { "layout": { "type": "grid", "minimumColumnWidth": "190px" } }
+      "attrs": { "layout": null, "className": "client-strip blocks-engine-css-owned-layout blocks-engine-css-owned-flow" }
     },
     { "path": "blocks.0.innerBlocks.0", "name": "core/paragraph" },
     { "path": "blocks.0.innerBlocks.3", "name": "core/paragraph" }
@@ -30,7 +30,8 @@
     { "path": "fallbacks", "assert": "count", "count": 0 },
     { "path": "blocks.0.innerBlocks", "assert": "count", "count": 4 },
     { "path": "blocks.0.attrs.style.spacing.blockGap", "assert": "equals", "value": null },
-    { "path": "serialized_blocks", "assert": "contains", "value": "is-layout-grid" },
-    { "path": "serialized_blocks", "assert": "contains", "value": "<p class=\"blocks-engine-inline-layout-carrier\"><span class=\"client\">Acme Corp</span></p>" }
+    { "path": "serialized_blocks", "assert": "not_contains", "value": "is-layout-grid" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "<p class=\"blocks-engine-inline-layout-carrier\"><span class=\"client\">Acme Corp</span></p>" },
+    { "path": "assets.1.content", "assert": "contains", "value": ".client-strip{display:grid;grid-template-columns:repeat(auto-fit, minmax(190px, 1fr));gap:24px}" }
   ]
 }
diff --git a/php-transformer/tests/fixtures/parity/html-autofit-grid-maps-to-minimum-column-width.json b/php-transformer/tests/fixtures/parity/html-autofit-grid-maps-to-minimum-column-width.json
deleted file mode 100644
index a04ba5a4..00000000
--- a/php-transformer/tests/fixtures/parity/html-autofit-grid-maps-to-minimum-column-width.json
+++ /dev/null
@@ -1,34 +0,0 @@
-{
-  "schema": "blocks-engine/php-transformer/parity-fixture/v1",
-  "name": "html-autofit-grid-maps-to-minimum-column-width",
-  "description": "An author-CSS grid container using repeat(auto-fit, minmax(W, 1fr)) is exactly expressible as native WordPress grid layout. It must become a core/group with layout {type:grid, minimumColumnWidth:W} instead of demoting to the css-owned flow group, which stacks the cards in a single column and loses the responsive multi-column arrangement.",
-  "source_reference": {
-    "repo": "php-transformer",
-    "path": "tests/fixtures/parity/html-autofit-grid-maps-to-minimum-column-width.json",
-    "notes": "Derived from a portfolio homepage where .work-grid { display:grid; grid-template-columns:repeat(auto-fit, minmax(240px, 1fr)) } collapsed to a vertical stack after transform: the css-owned demotion dropped the layout attribute and the carried CSS never reproduced the auto-fit tracks."
-  },
-  "legacy_comparison": {
-    "skip": true,
-    "reason": "Covers current PHP transformer layout classification behavior; no downstream legacy comparison."
-  },
-  "operation": "html_transformer.transform",
-  "input": {
-    "content": "<html><head><style>.work-grid{display:grid;grid-template-columns:repeat(auto-fit, minmax(240px, 1fr));gap:24px}.work-card h3{margin:0}</style></head><body><section class=\"work-grid\"><div class=\"work-card\"><h3>Ledger</h3><p>Design system for a fintech team.</p></div><div class=\"work-card\"><h3>Atlas</h3><p>Mapping tools for field research.</p></div><div class=\"work-card\"><h3>Relay</h3><p>Realtime dashboard for dispatch.</p></div></section></body></html>"
-  },
-  "expected_blocks": [
-    {
-      "path": "blocks.0",
-      "name": "core/group",
-      "attrs": { "layout": { "type": "grid", "minimumColumnWidth": "240px" } }
-    }
-  ],
-  "expected_fallbacks": [],
-  "expect": [
-    { "path": "status", "assert": "equals", "value": "success" },
-    { "path": "fallbacks", "assert": "count", "count": 0 },
-    { "path": "blocks.0.innerBlocks", "assert": "count", "count": 3 },
-    { "path": "serialized_blocks", "assert": "contains", "value": "is-layout-grid" },
-    { "path": "serialized_blocks", "assert": "contains", "value": "\"minimumColumnWidth\":\"240px\"" },
-    { "path": "serialized_blocks", "assert": "not_contains", "value": "blocks-engine-css-owned-flow" }
-  ]
-}
diff --git a/php-transformer/tests/fixtures/parity/html-autofit-grid-stays-css-owned.json b/php-transformer/tests/fixtures/parity/html-autofit-grid-stays-css-owned.json
new file mode 100644
index 00000000..5292d58f
--- /dev/null
+++ b/php-transformer/tests/fixtures/parity/html-autofit-grid-stays-css-owned.json
@@ -0,0 +1,36 @@
+{
+  "schema": "blocks-engine/php-transformer/parity-fixture/v1",
+  "name": "html-autofit-grid-stays-css-owned",
+  "description": "An author-CSS grid container using repeat(auto-fit, minmax(W, 1fr)) is NOT expressible as native WordPress grid layout: wp-includes/block-supports/layout.php hardcodes auto-fill in every branch that renders minimumColumnWidth, and auto-fill retains the tracks auto-fit collapses. The container must take the css-owned-GRID path, where the author rule keeps owning the exact authored track list — not the css-owned-FLOW demotion, which drops display:grid and stacks the cards in a single column.",
+  "source_reference": {
+    "repo": "php-transformer",
+    "path": "tests/fixtures/parity/html-autofit-grid-stays-css-owned.json",
+    "notes": "Derived from a portfolio homepage where .work-grid { display:grid; grid-template-columns:repeat(auto-fit, minmax(240px, 1fr)) } collapsed to a vertical stack after transform, because the demotion at the time dropped display:grid along with the layout attribute. Converting to a native layout attribute traded that stack for a different wrong geometry: WordPress rendered the tracks as auto-fill, retaining empty tracks and squeezing the cards into part of the measure. The css-owned-grid carrier is the path that reproduces the authored tracks exactly."
+  },
+  "legacy_comparison": {
+    "skip": true,
+    "reason": "Covers current PHP transformer layout classification behavior; no downstream legacy comparison."
+  },
+  "operation": "html_transformer.transform",
+  "input": {
+    "content": "<html><head><style>.work-grid{display:grid;grid-template-columns:repeat(auto-fit, minmax(240px, 1fr));gap:24px}.work-card h3{margin:0}</style></head><body><section class=\"work-grid\"><div class=\"work-card\"><h3>Ledger</h3><p>Design system for a fintech team.</p></div><div class=\"work-card\"><h3>Atlas</h3><p>Mapping tools for field research.</p></div><div class=\"work-card\"><h3>Relay</h3><p>Realtime dashboard for dispatch.</p></div></section></body></html>"
+  },
+  "expected_blocks": [
+    {
+      "path": "blocks.0",
+      "name": "core/group",
+      "attrs": { "layout": null, "className": "work-grid blocks-engine-css-owned-layout blocks-engine-css-owned-grid" }
+    }
+  ],
+  "expected_fallbacks": [],
+  "expect": [
+    { "path": "status", "assert": "equals", "value": "success" },
+    { "path": "fallbacks", "assert": "count", "count": 0 },
+    { "path": "blocks.0.innerBlocks", "assert": "count", "count": 3 },
+    { "path": "serialized_blocks", "assert": "not_contains", "value": "is-layout-grid" },
+    { "path": "serialized_blocks", "assert": "not_contains", "value": "minimumColumnWidth" },
+    { "path": "serialized_blocks", "assert": "not_contains", "value": "blocks-engine-css-owned-flow" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "blocks-engine-css-owned-grid" },
+    { "path": "assets.1.content", "assert": "contains", "value": ".work-grid{display:grid;grid-template-columns:repeat(auto-fit, minmax(240px, 1fr));gap:24px}" }
+  ]
+}
diff --git a/php-transformer/tests/fixtures/parity/html-horizontal-flex-stays-columns.json b/php-transformer/tests/fixtures/parity/html-horizontal-flex-stays-columns.json
index 5502a54b..db6cd94c 100644
--- a/php-transformer/tests/fixtures/parity/html-horizontal-flex-stays-columns.json
+++ b/php-transformer/tests/fixtures/parity/html-horizontal-flex-stays-columns.json
@@ -5,7 +5,7 @@
   "source_reference": {
     "repo": "php-transformer",
     "path": "tests/fixtures/parity/html-horizontal-flex-stays-columns.json",
-    "notes": "Same display:flex container shape as the vertical fixture but without flex-direction:column. The ColumnsPattern flex-direction guard does not fire, so the horizontal columns layout is preserved unchanged."
+    "notes": "Same display:flex container shape as the vertical fixture but without flex-direction:column. The ColumnsPattern flex-direction guard does not fire, so the horizontal columns layout is preserved unchanged. The container is demoted to a css-owned core/group, which drops the native layout attribute, so the inline display:flex must ride to the generated stylesheet on a geometry carrier or the two columns stack."
   },
   "legacy_comparison": {
     "skip": true,
@@ -28,7 +28,9 @@
     { "path": "status", "assert": "equals", "value": "success" },
     { "path": "blocks", "assert": "count", "count": 1 },
     { "path": "blocks.0.innerBlocks", "assert": "count", "count": 2 },
-    { "path": "serialized_blocks", "assert": "contains", "value": "cols blocks-engine-css-owned-layout" },
-    { "path": "serialized_blocks", "assert": "not_contains", "value": "blocks-engine/author-layout" }
+    { "path": "serialized_blocks", "assert": "contains", "value": "class=\"wp-block-group cols be-inline-geometry-" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "blocks-engine-css-owned-layout" },
+    { "path": "serialized_blocks", "assert": "not_contains", "value": "blocks-engine/author-layout" },
+    { "path": "assets.0.content", "assert": "contains", "value": "{display:flex !important}" }
   ]
 }
diff --git a/php-transformer/tests/fixtures/parity/html-single-child-flex-svg-address.json b/php-transformer/tests/fixtures/parity/html-single-child-flex-svg-address.json
index 63233023..f5032681 100644
--- a/php-transformer/tests/fixtures/parity/html-single-child-flex-svg-address.json
+++ b/php-transformer/tests/fixtures/parity/html-single-child-flex-svg-address.json
@@ -1,7 +1,7 @@
 {
   "schema": "blocks-engine/php-transformer/parity-fixture/v1",
   "name": "html-single-child-flex-svg-address",
-  "description": "Preserves single-child flex wrappers with passive illustrative inline SVG represented as native core/image and readable address line breaks without unsupported fallbacks.",
+  "description": "Preserves single-child flex wrappers with passive illustrative inline SVG represented as native core/image and readable address line breaks without unsupported fallbacks. The wrapper is demoted to a css-owned core/group, so its inline display:flex and centering must ride to the generated stylesheet on a geometry carrier — dropping them leaves the illustration flush left instead of centered.",
   "source_reference": {
     "repo": "php-transformer",
     "path": "tests/fixtures/parity/html-single-child-flex-svg-address.json",
@@ -17,7 +17,7 @@
   },
   "expected_blocks": [
     { "path": "blocks.0", "name": "core/group" },
-    { "path": "blocks.0.innerBlocks.0", "name": "core/group", "attrs": { "className": "illustration-frame blocks-engine-css-owned-layout" } },
+    { "path": "blocks.0.innerBlocks.0", "name": "core/group" },
     { "path": "blocks.0.innerBlocks.0.innerBlocks.0", "name": "core/image" },
     { "path": "blocks.0.innerBlocks.1", "name": "core/paragraph", "attrs": { "className": "contact-card", "content": "214 Maple Street<br>Benton Harbor, MI<br><a href=\"tel:+12695550193\">(269) 555-0193</a>" } }
   ],
@@ -27,7 +27,10 @@
     { "path": "fallbacks", "assert": "count", "count": 0 },
     { "path": "blocks.0.innerBlocks.0.innerBlocks.0.attrs.url", "assert": "contains", "value": "assets/materialized-svg/" },
     { "path": "blocks.0.innerBlocks.0.innerBlocks.0.attrs.alt", "assert": "equals", "value": "Storefront illustration" },
-    { "path": "assets", "assert": "count", "count": 1 },
+    { "path": "assets", "assert": "count", "count": 2 },
+    { "path": "assets.1.content", "assert": "contains", "value": "{display:flex !important;align-items:center !important;justify-content:center !important}" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "class=\"wp-block-group illustration-frame be-inline-geometry-" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "blocks-engine-css-owned-layout" },
     { "path": "serialized_blocks", "assert": "contains", "value": "Storefront illustration" },
     { "path": "serialized_blocks", "assert": "not_contains", "value": "blocks-engine/author-layout" },
     { "path": "coverage.0.fallback_count", "assert": "equals", "value": 0 }
diff --git a/php-transformer/tests/fixtures/parity/html-single-child-flex-svg.json b/php-transformer/tests/fixtures/parity/html-single-child-flex-svg.json
index ffadf039..c791d3a8 100644
--- a/php-transformer/tests/fixtures/parity/html-single-child-flex-svg.json
+++ b/php-transformer/tests/fixtures/parity/html-single-child-flex-svg.json
@@ -1,7 +1,7 @@
 {
   "schema": "blocks-engine/php-transformer/parity-fixture/v1",
   "name": "html-single-child-flex-svg",
-  "description": "Converts a single illustrative SVG inside a flex centering wrapper without leaking failed column-probe fallbacks, materializing the SVG as editable core/image media.",
+  "description": "Converts a single illustrative SVG inside a flex centering wrapper without leaking failed column-probe fallbacks, materializing the SVG as editable core/image media. The wrapper is demoted to a css-owned core/group, so its inline display:flex and justify-content must ride to the generated stylesheet on a geometry carrier — dropping them leaves the diagram flush left instead of centered.",
   "source_reference": {
     "repo": "php-transformer",
     "path": "tests/fixtures/parity/html-single-child-flex-svg.json",
@@ -27,8 +27,10 @@
     { "path": "blocks.0.innerBlocks", "assert": "count", "count": 1 },
     { "path": "blocks.0.innerBlocks.0.attrs.url", "assert": "contains", "value": "assets/materialized-svg/" },
     { "path": "assets.0.content", "assert": "contains", "value": "GIRTH" },
-    { "path": "assets", "assert": "count", "count": 1 },
-    { "path": "serialized_blocks", "assert": "contains", "value": "wp-block-group blocks-engine-css-owned-layout" },
+    { "path": "assets", "assert": "count", "count": 2 },
+    { "path": "assets.1.content", "assert": "contains", "value": "{display:flex !important;justify-content:center !important}" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "wp-block-group be-inline-geometry-" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "blocks-engine-css-owned-layout" },
     { "path": "serialized_blocks", "assert": "not_contains", "value": "blocks-engine/author-layout" },
     { "path": "fallbacks", "assert": "count", "count": 0 },
     { "path": "source_reports.conversion_report.metrics.fallback_count", "assert": "equals", "value": 0 }
diff --git a/php-transformer/tests/fixtures/parity/html-vertical-flex-column-becomes-group.json b/php-transformer/tests/fixtures/parity/html-vertical-flex-column-becomes-group.json
index 0f405947..5da9f5a1 100644
--- a/php-transformer/tests/fixtures/parity/html-vertical-flex-column-becomes-group.json
+++ b/php-transformer/tests/fixtures/parity/html-vertical-flex-column-becomes-group.json
@@ -5,7 +5,7 @@
   "source_reference": {
     "repo": "php-transformer",
     "path": "tests/fixtures/parity/html-vertical-flex-column-becomes-group.json",
-    "notes": "Keys solely off the CSS flex-direction:column value (never class/fixture names). The author-layout companion preserves source CSS as the sole layout authority."
+    "notes": "Keys solely off the CSS flex-direction:column value (never class/fixture names). The author-layout companion preserves source CSS as the sole layout authority. The css-owned demotion drops the native layout attribute, so the inline flex declarations ride to the generated stylesheet on a geometry carrier: a flow stack only coincidentally resembles a flex column, and loses the authored gap and any cross-axis alignment."
   },
   "legacy_comparison": {
     "skip": true,
@@ -16,7 +16,7 @@
     "content": "<div class=\"hero\" style=\"display:flex;flex-direction:column;gap:1rem\"><span class=\"eyebrow\">New</span><h1>Title</h1><p>Subhead</p><a class=\"btn\" href=\"/x\" style=\"padding:10px 16px;background:#135e96\">Go</a></div>"
   },
   "expected_blocks": [
-    { "path": "blocks.0", "name": "core/group", "attrs": { "className": "hero blocks-engine-css-owned-layout blocks-engine-css-owned-flow" } },
+    { "path": "blocks.0", "name": "core/group" },
     { "path": "blocks.0.innerBlocks.0", "name": "core/paragraph" },
     { "path": "blocks.0.innerBlocks.1", "name": "core/heading", "attrs": { "content": "Title", "level": 1 } },
     { "path": "blocks.0.innerBlocks.2", "name": "core/paragraph", "attrs": { "content": "Subhead" } },
@@ -26,7 +26,9 @@
     { "path": "status", "assert": "equals", "value": "success" },
     { "path": "blocks", "assert": "count", "count": 1 },
     { "path": "blocks.0.innerBlocks", "assert": "count", "count": 4 },
-    { "path": "serialized_blocks", "assert": "contains", "value": "<div class=\"wp-block-group hero blocks-engine-css-owned-layout blocks-engine-css-owned-flow\">" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "<div class=\"wp-block-group hero be-inline-geometry-" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "blocks-engine-css-owned-layout blocks-engine-css-owned-flow\">" },
+    { "path": "assets.0.content", "assert": "contains", "value": "{display:flex !important;flex-direction:column !important;gap:1rem !important}" },
     { "path": "serialized_blocks", "assert": "contains", "value": "wp-block-button__link" },
     { "path": "serialized_blocks", "assert": "not_contains", "value": "blocks-engine/author-layout" },
     { "path": "serialized_blocks", "assert": "not_contains", "value": "wp:columns" }
diff --git a/php-transformer/tests/unit/block-style-support-conversion.php b/php-transformer/tests/unit/block-style-support-conversion.php
index e81a26f3..fec27c7d 100644
--- a/php-transformer/tests/unit/block-style-support-conversion.php
+++ b/php-transformer/tests/unit/block-style-support-conversion.php
@@ -82,7 +82,7 @@
 $assert(! isset($groupAttrs['style']['spacing']['blockGap']), '15: core group save omits block gap without a core layout attribute', json_encode($groupAttrs));
 $assert(str_contains($groupInnerHtml, 'min-height:100svh'), '16: core group retains supported dimensions', $groupInnerHtml);
 
-$nativeGridHtml = '<div class="tpl-grid" style="display:grid;grid-template-columns:repeat(auto-fit, minmax(290px, 1fr));gap:1.2rem"><p>Fallback card</p></div>';
+$nativeGridHtml = '<div class="tpl-grid" style="display:grid;grid-template-columns:repeat(auto-fill, minmax(290px, 1fr));gap:1.2rem"><p>Fallback card</p></div>';
 $nativeGridResult = ( new HtmlTransformer() )->transform($nativeGridHtml, array())->toArray();
 $nativeGrid = $nativeGridResult['blocks'][0] ?? array();
 $nativeGridAttrs = is_array($nativeGrid['attrs'] ?? null) ? $nativeGrid['attrs'] : array();
@@ -94,6 +94,17 @@
 $assert(! str_contains($nativeGridMarkup, 'gap:1.2rem'), '16c: native Group grid markup omits inline gap that Gutenberg save does not reproduce', $nativeGridMarkup);
 $assert(str_contains($nativeGridCss, 'gap:1.2rem !important'), '16d: inline native Group grid gap moves to the generated geometry carrier', $nativeGridCss);
 
+// The same shape authored with auto-fit is NOT natively expressible: core's
+// layout support hardcodes auto-fill, which retains the tracks auto-fit
+// collapses. See tests/unit/auto-fit-grid-carrier.php for the full contract.
+$autoFitGridResult = ( new HtmlTransformer() )->transform(
+    '<div class="tpl-grid" style="display:grid;grid-template-columns:repeat(auto-fit, minmax(290px, 1fr));gap:1.2rem"><p>Fallback card</p></div>',
+    array()
+)->toArray();
+$autoFitGridAttrs = is_array($autoFitGridResult['blocks'][0]['attrs'] ?? null) ? $autoFitGridResult['blocks'][0]['attrs'] : array();
+
+$assert(! isset($autoFitGridAttrs['layout']), '16e: the same shape authored with auto-fit stays under CSS ownership', json_encode($autoFitGridAttrs));
+
 $unorderedListSource = '<ul style="list-style:none"><li>Alpha</li></ul>';
 $unorderedListResult = ( new HtmlTransformer() )->transform($unorderedListSource)->toArray();
 $unorderedListAttrs = $unorderedListResult['blocks'][0]['attrs'] ?? array();

From 513c30725b5a684c15e56dab4aa0566c75d138fa Mon Sep 17 00:00:00 2001
From: Matthew Batchelder <borkweb@gmail.com>
Date: Sun, 16 Aug 2026 10:27:53 -0700
Subject: [PATCH 3/4] test(php-transformer): freeze the button font-family
 carry contract

## Summary
Adds a deliberately RED contract proving an authored `font-family` on a button-mapped rule must reach the core/button block attributes, and that the other carried declarations keep mapping exactly as they do today.

## Why
`ButtonStyleResolver::BUTTON_TYPOGRAPHY` projects `fontSize`, `fontWeight`, `letterSpacing`, `lineHeight` and `textTransform` onto buttons and drops `font-family`. core/button registers a `fontFamily` attribute, which core injects only when the typography fontFamily support is enabled, so this is a dropped declaration rather than an unsupported one.

The drop is not benign. The authored `.btn` class is consumed into block attributes, so the surviving author rule is rewritten behind `:where()` at zero specificity and matches nothing that can win. theme.json's `styles.elements.button.typography.fontFamily` then substitutes the design-direction typeface, which widens the measure enough to wrap hero CTAs onto two lines.

## How
`tests/unit/button-font-family-carry.php` asserts the support exists before relying on it, pins the resolver's complete attribute output so no other carried declaration can drift, covers a rule that authors no font-family, and repeats the end-to-end shape from the report where the class is consumed.

## Testing
RED evidence at this commit: `php tests/unit/button-font-family-carry.php` exits 1 with 5 failed and 4 passed assertions. The passing four include the support check, which must hold before the fix is safe to write.
---
 php-transformer/composer.json                 |   1 +
 .../tests/unit/button-font-family-carry.php   | 144 ++++++++++++++++++
 2 files changed, 145 insertions(+)
 create mode 100644 php-transformer/tests/unit/button-font-family-carry.php

diff --git a/php-transformer/composer.json b/php-transformer/composer.json
index 5a8a61a0..a49b7213 100644
--- a/php-transformer/composer.json
+++ b/php-transformer/composer.json
@@ -87,6 +87,7 @@
       "php tests/unit/navigation-underline-color-resolver.php",
       "php tests/unit/button-signal-classifier.php",
       "php tests/unit/button-style-resolver.php",
+      "php tests/unit/button-font-family-carry.php",
       "php tests/unit/button-visual-probe-diagnostics.php",
       "php tests/unit/shell-landmark-policy.php",
       "php tests/unit/block-style-support-conversion.php",
diff --git a/php-transformer/tests/unit/button-font-family-carry.php b/php-transformer/tests/unit/button-font-family-carry.php
new file mode 100644
index 00000000..6c31484d
--- /dev/null
+++ b/php-transformer/tests/unit/button-font-family-carry.php
@@ -0,0 +1,144 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Contract for an authored font-family on a button-mapped rule.
+ *
+ * core/button registers a `fontFamily` attribute (see
+ * packages/blocks-engine/src/core-block-attrs.json, generated from
+ * @wordpress/block-library), which core only injects when the typography
+ * fontFamily support is enabled. A raw authored value is not a preset slug, so
+ * it belongs in `style.typography.fontFamily` alongside the other carried
+ * typography declarations.
+ *
+ * Dropping it is not benign: the authored `.btn` class is consumed into block
+ * attributes, so the surviving author rule matches nothing at a specificity that
+ * can win, and theme.json's styles.elements.button.typography.fontFamily
+ * substitutes the design-direction typeface.
+ */
+
+require dirname(__DIR__, 2) . '/vendor/autoload.php';
+
+use Automattic\BlocksEngine\PhpTransformer\HtmlToBlocks\HtmlTransformer;
+use Automattic\BlocksEngine\PhpTransformer\HtmlToBlocks\Patterns\ButtonStyleResolver;
+
+$failures = 0;
+$passes = 0;
+
+$assert = static function (bool $condition, string $message, string $detail = '') use (&$failures, &$passes): void {
+    if ( $condition ) {
+        ++$passes;
+        return;
+    }
+
+    ++$failures;
+    fwrite(STDERR, 'FAIL: ' . $message . ('' !== $detail ? ' - ' . $detail : '') . PHP_EOL);
+};
+
+// -- G11: the support this fix relies on must actually exist.
+$blockAttrs = json_decode(
+    (string) file_get_contents(dirname(__DIR__, 3) . '/packages/blocks-engine/src/core-block-attrs.json'),
+    true
+);
+$buttonAttributes = is_array($blockAttrs['blocks']['core/button'] ?? null) ? $blockAttrs['blocks']['core/button'] : array();
+
+$assert(
+    in_array('fontFamily', $buttonAttributes, true),
+    'core/button registers a fontFamily attribute, so the support is real',
+    implode(',', $buttonAttributes)
+);
+
+// -- G9 + G10 at the resolver: font-family joins the carried set, and every
+// other carried declaration keeps mapping byte-for-byte as it does today.
+$authoredButtonCss = 'background:#135e96;color:#ffffff;border:2px solid #0b3d6b;border-radius:6px;'
+    . 'font-family:"Trebuchet MS", "Segoe UI", sans-serif;font-size:0.88rem;font-weight:700;'
+    . 'letter-spacing:0.08em;text-transform:uppercase;padding:10px 16px';
+
+$resolved = ( new ButtonStyleResolver() )->nativeAttributes($authoredButtonCss);
+$expected = array(
+    'style' => array(
+        'color' => array( 'background' => '#135e96', 'text' => '#ffffff' ),
+        'border' => array( 'width' => '2px', 'style' => 'solid', 'color' => '#0b3d6b', 'radius' => '6px' ),
+        'spacing' => array( 'padding' => array( 'top' => '10px', 'right' => '16px', 'bottom' => '10px', 'left' => '16px' ) ),
+        'typography' => array(
+            'fontFamily' => '"Trebuchet MS", "Segoe UI", sans-serif',
+            'fontSize' => '0.88rem',
+            'fontWeight' => '700',
+            'letterSpacing' => '0.08em',
+            'textTransform' => 'uppercase',
+        ),
+    ),
+);
+
+$assert(
+    '"Trebuchet MS", "Segoe UI", sans-serif' === (string) ($resolved['style']['typography']['fontFamily'] ?? ''),
+    'resolver: the authored font-family reaches style.typography.fontFamily',
+    json_encode($resolved['style']['typography'] ?? null)
+);
+$assert(
+    json_encode($expected) === json_encode($resolved),
+    'resolver: every carried declaration maps exactly, with font-family in canonical order',
+    json_encode($resolved)
+);
+
+// -- A button whose rule authors no font-family gains none.
+$withoutFamily = ( new ButtonStyleResolver() )->nativeAttributes('background:#135e96;color:#ffffff;font-size:0.88rem');
+
+$assert(
+    ! isset($withoutFamily['style']['typography']['fontFamily']),
+    'resolver: no font-family is invented when the rule authors none',
+    json_encode($withoutFamily)
+);
+$assert(
+    array( 'fontSize' => '0.88rem' ) === ($withoutFamily['style']['typography'] ?? null),
+    'resolver: the remaining typography subset is untouched',
+    json_encode($withoutFamily['style']['typography'] ?? null)
+);
+
+// -- G9 end to end: the shape from the report, where `.btn` is consumed into
+// block attributes and only the block attribute can still carry the typeface.
+$result = ( new HtmlTransformer() )->transform(
+    '<style>.btn{background:#135e96;color:#fff;border:2px solid #0b3d6b;border-radius:6px;'
+    . 'font-family:"Trebuchet MS", "Segoe UI", sans-serif;font-size:0.88rem;font-weight:700;'
+    . 'letter-spacing:0.08em;text-transform:uppercase;padding:10px 16px}</style>'
+    . '<div class="hero-cta"><a class="btn" href="/a">Start</a><a class="btn" href="/b">Learn</a></div>',
+    array()
+)->toArray();
+$button = $result['blocks'][0]['innerBlocks'][0] ?? array();
+$buttonTypography = $button['attrs']['style']['typography'] ?? null;
+$serialized = (string) ($result['serialized_blocks'] ?? '');
+
+$assert(
+    'core/button' === ($button['blockName'] ?? ''),
+    'end to end: the authored anchor still becomes a core/button',
+    (string) ($button['blockName'] ?? '(none)')
+);
+$assert(
+    '"Trebuchet MS", "Segoe UI", sans-serif' === (string) ($buttonTypography['fontFamily'] ?? ''),
+    'end to end: the block attributes carry the authored typeface',
+    json_encode($buttonTypography)
+);
+$assert(
+    str_contains($serialized, 'font-family:&quot;Trebuchet MS&quot;, &quot;Segoe UI&quot;, sans-serif')
+        || str_contains($serialized, 'font-family:"Trebuchet MS", "Segoe UI", sans-serif'),
+    'end to end: the rendered button link declares the authored typeface inline, where theme.json cannot outrank it',
+    $serialized
+);
+$assert(
+    array(
+        'fontFamily' => '"Trebuchet MS", "Segoe UI", sans-serif',
+        'fontSize' => '0.88rem',
+        'fontWeight' => '700',
+        'letterSpacing' => '0.08em',
+        'textTransform' => 'uppercase',
+    ) === $buttonTypography,
+    'end to end: the other carried typography declarations are unchanged',
+    json_encode($buttonTypography)
+);
+
+if ( $failures > 0 ) {
+    fwrite(STDERR, "Button font-family carry contract: {$failures} failed, {$passes} passed\n");
+    exit(1);
+}
+
+fwrite(STDOUT, "Button font-family carry contract passed: {$passes} assertions\n");

From f4edf0a6b51e7fdd0378aae56a7056ebe65e5bf3 Mon Sep 17 00:00:00 2001
From: Matthew Batchelder <borkweb@gmail.com>
Date: Sun, 16 Aug 2026 10:30:03 -0700
Subject: [PATCH 4/4] fix(php-transformer): carry an authored font-family onto
 core/button
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

## Summary
Adds `fontFamily` to the typography subset projected onto core/button, and to the save-markup serializer that renders those attributes.

## Why
`ButtonStyleResolver` carried a button rule's background, text color, border, radius, font-size, weight, letter-spacing and text-transform, but dropped `font-family`. core/button registers a `fontFamily` attribute, which core injects only when the typography fontFamily support is enabled, so the declaration was expressible all along.

Because the authored class is consumed into block attributes, its surviving rule is rewritten behind `:where()` at zero specificity and cannot win. theme.json's `styles.elements.button.typography.fontFamily` therefore substituted the design-direction typeface, widening hero CTAs enough to wrap them onto two lines.

## How
A raw authored family is a custom value rather than a preset slug, so it rides in `style.typography.fontFamily`. `BlockFactory` did not map that attribute either, so the block attribute alone would not have reached the rendered link; the serializer now emits `font-family` inline the way core's style engine does, ahead of the other typography declarations. Inline style outranks theme.json's element styles, so no `!important` bridge is needed and none was added.

## Testing
- [ ] `composer test:unit` — 0 failures across 39 scripts.
- [ ] `composer test` — 0 failures across 51 scripts, 275 parity fixtures, none edited for this defect.
- [ ] `php tests/unit/button-font-family-carry.php` — 9 assertions, up from 4 passing and 5 failing at the RED commit.
---
 php-transformer/src/HtmlToBlocks/BlockFactory.php      |  3 +++
 .../src/HtmlToBlocks/Patterns/ButtonStyleResolver.php  | 10 +++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/php-transformer/src/HtmlToBlocks/BlockFactory.php b/php-transformer/src/HtmlToBlocks/BlockFactory.php
index 9fc0b616..725a9ebd 100644
--- a/php-transformer/src/HtmlToBlocks/BlockFactory.php
+++ b/php-transformer/src/HtmlToBlocks/BlockFactory.php
@@ -979,6 +979,9 @@ private function buttonStyleSupport(array $attrs): array
             $classes[] = 'has-custom-font-size';
         }
         $typographyMap = array(
+            // A raw authored family is a custom value, so core's style engine
+            // serializes it inline rather than as a has-*-font-family class.
+            'fontFamily'    => 'font-family',
             'fontSize'      => 'font-size',
             'fontWeight'    => 'font-weight',
             'letterSpacing' => 'letter-spacing',
diff --git a/php-transformer/src/HtmlToBlocks/Patterns/ButtonStyleResolver.php b/php-transformer/src/HtmlToBlocks/Patterns/ButtonStyleResolver.php
index 17909afd..f98b8459 100644
--- a/php-transformer/src/HtmlToBlocks/Patterns/ButtonStyleResolver.php
+++ b/php-transformer/src/HtmlToBlocks/Patterns/ButtonStyleResolver.php
@@ -34,8 +34,16 @@ final class ButtonStyleResolver
 {
     /**
      * Typography supports projected onto buttons, in canonical emission order.
+     *
+     * fontFamily belongs here: core/button registers a `fontFamily` attribute,
+     * which core injects only when the typography fontFamily support is enabled.
+     * A raw authored value is not a preset slug, so it rides in
+     * style.typography.fontFamily and serializes inline on the link. Dropping it
+     * left the typeface to theme.json's styles.elements.button, because the
+     * authored class is consumed into block attributes and its rewritten rule no
+     * longer wins the cascade.
      */
-    private const BUTTON_TYPOGRAPHY = array( 'fontSize', 'fontWeight', 'letterSpacing', 'lineHeight', 'textTransform' );
+    private const BUTTON_TYPOGRAPHY = array( 'fontFamily', 'fontSize', 'fontWeight', 'letterSpacing', 'lineHeight', 'textTransform' );
 
     private readonly StyleAttributeMapper $mapper;