diff --git a/resources/css/components.css b/resources/css/components.css
index 48216f2d..6abf51ab 100644
--- a/resources/css/components.css
+++ b/resources/css/components.css
@@ -57,5 +57,6 @@
 
 /* Containment */
 @import './components/divider.css';
+@import './components/collapse.css';
 
 /* Navigation */
diff --git a/resources/css/components/collapse.css b/resources/css/components/collapse.css
index 0ba2d2cf..492f2286 100644
--- a/resources/css/components/collapse.css
+++ b/resources/css/components/collapse.css
@@ -1,22 +1,42 @@
 /*
- * `<x-collapse>`'s height, eased open.
+ * <x-collapse>: not an M3 component — M3 has expandable list items and menu expansion but no
+ * standalone disclosure — built on the native `<details>` so it opens without script, keeps its
+ * state through a Livewire morph, and is announced as a disclosure by the browser. Judged against
+ * foundations rather than any M3 component spec (docs/audits/m3-alignment/containment.md, C-24).
  *
- * The native `<details>` opens and closes by itself; what it cannot do on its own is animate, so
- * the component sets `interpolate-size: allow-keywords` (which makes `0` → `auto` an animatable
- * pair) and this transitions `block-size` on `::details-content`, the pseudo-element that holds
- * everything after the `<summary>`. `content-visibility` goes with it, `allow-discrete`, so the
- * content stays rendered while it closes rather than vanishing on the first frame.
+ * `interpolate-size: allow-keywords` makes `<details>`'s `0 -> auto` block-size an animatable pair,
+ * so `::details-content` (the pseudo-element that holds everything after the summary) can transition
+ * `block-size` on the fast spatial spring, the same one that turns the chevron; `content-visibility`
+ * goes with it, `allow-discrete`, so the content stays rendered while it closes rather than
+ * vanishing on the first frame. Under reduced motion the duration token is zero
+ * (tokens/motion.css), so the section snaps open with nothing else to do.
  *
- * The fast spatial spring, as the chevron beside it: M3's spatial track is for anything that
- * changes size or position. Under reduced motion the duration token is zero
- * (resources/css/tokens/motion.css), so the section snaps open with nothing else to do.
+ * `data-md-variant="filled"` is a surface-container tile with a large corner; the summary inherits
+ * it (`border-radius: inherit`) so the shared `md-state-layer`'s own `::before`, which also
+ * inherits, draws inside the same shape. `md-state-layer` and `md-focus-ring`
+ * (foundation/interaction.css) sit on the summary; this file only refines their target — the
+ * 48px summary height, the icon and chevron colour — never their opacities or ring.
  *
- * Not an M3 component — M3 has expandable list items and menu expansion, no standalone disclosure
- * — so the geometry is the library's and only the motion tokens are M3's.
+ * Icon-to-title and title-to-chevron gaps are 12px, midway between two spacing steps
+ * (`space100` 8px, `space200` 16px); written as their average rather than a bare literal so the
+ * value still comes from the scale.
  */
 
-@layer components {
-    [data-collapse]::details-content {
+@layer material.reset, material.tokens, material.base, material.layout, material.components, material.text, material.visibility;
+
+@import './icon.css';
+
+@layer material.components {
+    [data-md-collapse] {
+        interpolate-size: allow-keywords;
+    }
+
+    [data-md-collapse][data-md-variant='filled'] {
+        border-radius: var(--md-sys-shape-corner-lg);
+        background-color: var(--md-sys-color-surface-container);
+    }
+
+    [data-md-collapse]::details-content {
         block-size: 0;
         overflow: hidden;
         transition:
@@ -24,7 +44,56 @@
             content-visibility var(--md-sys-motion-spatial-fast-duration) allow-discrete;
     }
 
-    [data-collapse][open]::details-content {
+    [data-md-collapse][open]::details-content {
         block-size: auto;
     }
+
+    [data-md-collapse-summary] {
+        display: flex;
+        min-block-size: var(--md-sys-measurement-space600);
+        align-items: center;
+        gap: calc((var(--md-sys-measurement-space100) + var(--md-sys-measurement-space200)) / 2);
+        border-radius: inherit;
+        cursor: pointer;
+        list-style-type: none;
+        font: var(--md-sys-typescale-title-md);
+        letter-spacing: var(--md-sys-typescale-title-md-tracking);
+
+        &::-webkit-details-marker {
+            display: none;
+        }
+    }
+
+    [data-md-collapse][data-md-variant='filled'] [data-md-collapse-summary] {
+        padding-inline: var(--md-sys-measurement-space200);
+    }
+
+    [data-md-collapse-icon] {
+        color: var(--md-sys-color-on-surface-variant);
+    }
+
+    [data-md-collapse-title] {
+        min-width: 0;
+        flex: 1 1 0%;
+    }
+
+    [data-md-collapse-chevron] {
+        color: var(--md-sys-color-on-surface-variant);
+        transition: rotate var(--md-sys-motion-spatial-fast-duration) var(--md-sys-motion-spatial-fast);
+    }
+
+    [data-md-collapse][open] [data-md-collapse-chevron] {
+        rotate: 180deg;
+    }
+
+    [data-md-collapse-body] {
+        padding-block: var(--md-sys-measurement-space50) var(--md-sys-measurement-space200);
+        color: var(--md-sys-color-on-surface-variant);
+        font: var(--md-sys-typescale-body-md);
+        letter-spacing: var(--md-sys-typescale-body-md-tracking);
+    }
+
+    [data-md-collapse][data-md-variant='filled'] [data-md-collapse-body] {
+        padding-inline: var(--md-sys-measurement-space200);
+    }
 }
diff --git a/resources/css/tailwind.css b/resources/css/tailwind.css
index d030938a..7e81645d 100644
--- a/resources/css/tailwind.css
+++ b/resources/css/tailwind.css
@@ -19,7 +19,6 @@
 
 /* Containment — leaving in step 36 */
 @import './components/list.css';
-@import './components/collapse.css';
 @import './components/dialog.css';
 
 /* Navigation — leaving in step 36 */
diff --git a/resources/views/components/collapse.blade.php b/resources/views/components/collapse.blade.php
index ec7ef45e..fe11a05e 100644
--- a/resources/views/components/collapse.blade.php
+++ b/resources/views/components/collapse.blade.php
@@ -4,10 +4,10 @@
      state through a Livewire morph (`wire:ignore.self` stops the server's HTML from closing it),
      and is announced as a disclosure. Drawn in M3's terms: a title-medium `title` (or a
      `heading` slot), an optional leading `icon`, a chevron that turns over on the spatial spring,
-     and — where the browser supports animating `details` content (`interpolate-size`) — a height
-     that eases open on the same spring (`data-collapse`, resources/css/components/collapse.css).
-     `open` starts it open. `variant`: `plain` (the default, on the surface around
-     it) or `filled` (a surface-container tile with a large corner).
+     and a height that eases open on the same spring
+     (`data-md-collapse`, resources/css/components/collapse.css). `open` starts it open.
+     `variant`: `plain` (the default, on the surface around it) or `filled` (a surface-container
+     tile with a large corner).
 
      Its open state can be bound, both ways:
      - `wire:model` (any modifiers; `.live` tells the server at once) entangles it with a Livewire
@@ -18,7 +18,10 @@
        paint, until Alpine starts and applies the property.
      The state lives in `collapseOpen` on the `<details>`, a name kept clear of `open`, which a
      dialog inside the slot may be reading from a scope around it. Without a binding there is no
-     Alpine on it at all. --}}
+     Alpine on it at all.
+
+     It renders the foundation's `md-state-layer` and `md-focus-ring` (foundation/interaction.css)
+     on the summary; nothing here copies their rules. --}}
 
 @props([
     'title' => null,
@@ -31,6 +34,7 @@
     $model = $attributes->wire('model')->value() ?: null;
     $bound = $model !== null || count($attributes->whereStartsWith('x-model')->getAttributes()) > 0;
     $expanded = (bool) $open;
+    $variant = $variant === 'filled' ? 'filled' : 'plain';
 
     if ($model !== null && ($component = \Livewire\Livewire::current()) !== null) {
         $expanded = (bool) data_get($component, $model);
@@ -39,7 +43,8 @@
 
 <details
     wire:ignore.self
-    data-collapse
+    data-md-collapse
+    data-md-variant="{{ $variant }}"
     @if ($bound)
         x-data="{ collapseOpen: @if ($model !== null) @entangle($attributes->wire('model')) @else @js($expanded) @endif }"
         @if ($model === null) x-modelable="collapseOpen" @endif
@@ -47,25 +52,19 @@
         x-on:toggle="collapseOpen = $el.open"
     @endif
     @if ($expanded) open @endif
-    {{ $attributes->whereDoesntStartWith('wire:model')->class([
-        'group/collapse [interpolate-size:allow-keywords]',
-        'rounded-corner-lg bg-surface-container' => $variant === 'filled',
-    ]) }}
+    {{ $attributes->whereDoesntStartWith('wire:model') }}
 >
-    <summary @class([
-        'state-layer focus-ring flex min-h-12 cursor-pointer list-none items-center gap-3 rounded-[inherit] type-title-md [&::-webkit-details-marker]:hidden',
-        'px-4' => $variant === 'filled',
-    ])>
+    <summary data-md-collapse-summary class="md-state-layer md-focus-ring">
         @if ($icon)
-            <x-livewire-material::icon :name="$icon" class="size-6 text-on-surface-variant" />
+            <x-livewire-material::icon :name="$icon" data-md-collapse-icon />
         @endif
 
-        <span class="min-w-0 flex-1">{{ $heading ?? $title }}</span>
+        <span data-md-collapse-title>{{ $heading ?? $title }}</span>
 
-        <x-livewire-material::icon name="expand_more" class="size-6 text-on-surface-variant transition-[rotate] duration-(--md-sys-motion-spatial-fast-duration) ease-spatial-fast group-open/collapse:rotate-180" />
+        <x-livewire-material::icon name="expand_more" data-md-collapse-chevron />
     </summary>
 
-    <div @class(['pb-4 type-body-md text-on-surface-variant', 'px-4' => $variant === 'filled', 'pt-1'])>
+    <div data-md-collapse-body>
         {{ $slot }}
     </div>
 </details>
diff --git a/tests/Feature/Components/CollapseTest.php b/tests/Feature/Components/CollapseTest.php
index 8c04fbd9..f72e0cc1 100644
--- a/tests/Feature/Components/CollapseTest.php
+++ b/tests/Feature/Components/CollapseTest.php
@@ -10,24 +10,25 @@ it('discloses on the native details element, kept open through a morph', functio
         ->toContain('<details')
         ->toContain('wire:ignore.self')
         ->toContain(' open ')
-        ->toContain('rounded-corner-lg bg-surface-container')
-        ->toContain('data-collapse')
-        ->toContain('[interpolate-size:allow-keywords]')
+        ->toContain('data-md-collapse')
+        ->toContain('data-md-variant="filled"')
+        ->toContain('data-md-collapse-icon')
+        ->toContain('data-md-collapse-chevron')
         ->toContain('How long do links last?')
         ->toContain('Until the expiry.')
-        ->toContain('group-open/collapse:rotate-180');
+        ->toContain('class="md-state-layer md-focus-ring"');
 });
 
 it('has no Alpine on it without a binding', function () {
     $html = (string) $this->blade('<x-collapse title="Advanced" open>Body</x-collapse>');
 
     expect($html)
-        ->toMatch('/<details\s+wire:ignore.self\s+data-collapse\s+open\s+class="group\/collapse/')
+        ->toMatch('/<details\s+wire:ignore.self\s+data-md-collapse\s+data-md-variant="plain"\s+open\s+/')
         ->not->toContain('x-data')
         ->not->toContain('x-modelable')
         ->not->toContain('x-on:toggle')
         ->and((string) $this->blade('<x-collapse title="Advanced">Body</x-collapse>'))
-        ->toMatch('/<details\s+wire:ignore.self\s+data-collapse\s+class="group\/collapse/');
+        ->toMatch('/<details\s+wire:ignore.self\s+data-md-collapse\s+data-md-variant="plain"\s+/');
 });
 
 it('binds its open state through x-model, drawing the open prop until Alpine starts', function () {
diff --git a/tests/Feature/Components/ContainmentStylesheetsTest.php b/tests/Feature/Components/ContainmentStylesheetsTest.php
index 6af726e4..d7327f6d 100644
--- a/tests/Feature/Components/ContainmentStylesheetsTest.php
+++ b/tests/Feature/Components/ContainmentStylesheetsTest.php
@@ -17,6 +17,7 @@ use NoNameWeb\LivewireMaterial\Tests\Support\ViewClasses;
  */
 dataset('containment components', [
     'divider',
+    'collapse',
 ]);
 
 /**