diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0372f2e..f1b3512 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,13 @@ at boot. Entries start at 0.3.0.
 
 ## Unreleased
 
+### The tests fail on a stylesheet feature the browser floor lacks
+
+Every stylesheet in the repo, the examples' included, is checked against Chromium 87, Safari 15 and
+Firefox 115 using MDN's compat data; a feature one of them lacks passes only inside an `@supports`
+that names it. The core stylesheets now guard each such feature, which those engines skip instead
+of misreading. Pages render as before in current browsers.
+
 ### Every release reaches Docker Hub, and the Hub page has a short description
 
 Releases 0.4.1 to 0.4.20 were tagged in git but never published to Docker Hub or as semver tags in
diff --git a/README.md b/README.md
index c05f5e6..2c1b130 100644
--- a/README.md
+++ b/README.md
@@ -1059,9 +1059,11 @@ Chromium 87, Safari 15 and Firefox 115, the engines in years-old TVs, kiosks and
 features [Baseline Widely available](https://web-platform-dx.github.io/baseline/) (in every
 major engine for 30 months) may shape how a page looks; anything newer, like popovers or anchor
 positioning, only enhances. A feature the floor engines lack, such as `:has()`, may
-decorate a page but never decide whether something shows or can be read. A colour is hex, `rgb()` or
-`hsl()`, never `oklch()`: a custom property holding it passes unchecked, and the `var()` reading it
-then renders transparent.
+decorate a page but never decide whether something shows or can be read. The tests hold every
+stylesheet in `public/` and `examples/` to this floor, against MDN's compat data: a feature a floor
+engine lacks fails them unless it sits inside an `@supports` whose condition names it
+(`src/ui/stylesheet-floor.ts` lists what it checks). A plugin stylesheet mounted from outside the
+repo is not checked.
 
 **A field whose options depend on another field is asked for in steps, one GET form each**, so every
 choice is in the URL, and nothing typed is lost to one because nothing is typed until they are made.
diff --git a/docs/reading-cost.md b/docs/reading-cost.md
index e8ce3f5..853a87c 100644
--- a/docs/reading-cost.md
+++ b/docs/reading-cost.md
@@ -209,3 +209,20 @@ Read most (opens and searches):
 - `examples/plugins/admin/i18n/en-US.ts` 2
 - `CHANGELOG.md` 1
 - `examples/plugins/admin/admin-groups.test.ts` 1
+
+## Reading cost, chunk c90b25a50479, finished 2026-10-31T10:40:23Z
+
+90935 new input tokens before the first commit.
+
+Read most (opens and searches):
+
+- `README.md` 9
+- `todo.md` 5
+- `AGENTS.md` 3
+- `CHANGELOG.md` 1
+- `package.json` 1
+- `registry-cleanup/cleanup.ts` 1
+- `release-tooling/dockerhub-overview.md.tmpl` 1
+- `release-tooling/dockerhub-overview.test.ts` 1
+- `release-tooling/dockerhub-overview.ts` 1
+- `src/ci-gate.test.ts` 1
diff --git a/package-lock.json b/package-lock.json
index 4ecde74..ecfb573 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -12,6 +12,7 @@
         "postgres": "3.4.9"
       },
       "devDependencies": {
+        "@mdn/browser-compat-data": "8.1.7",
         "@types/ejs": "3.1.5",
         "@types/node": "24.19.2",
         "acorn": "8.19.0",
@@ -30,6 +31,13 @@
         "node": ">=18"
       }
     },
+    "node_modules/@mdn/browser-compat-data": {
+      "version": "8.1.7",
+      "resolved": "https://registry.npmjs.org/@mdn/browser-compat-data/-/browser-compat-data-8.1.7.tgz",
+      "integrity": "sha512-TJ+rX530bFoE8678PuH2gjCSzwpBcaLlcElRhnnxFdkLHHT1clUAxtjImPFZDAlWQ005px9xaJ4Nd+aQp/rylQ==",
+      "dev": true,
+      "license": "CC0-1.0"
+    },
     "node_modules/@types/ejs": {
       "version": "3.1.5",
       "resolved": "https://registry.npmjs.org/@types/ejs/-/ejs-3.1.5.tgz",
diff --git a/package.json b/package.json
index 7771026..20213c4 100644
--- a/package.json
+++ b/package.json
@@ -22,6 +22,7 @@
     "postgres": "3.4.9"
   },
   "devDependencies": {
+    "@mdn/browser-compat-data": "8.1.7",
     "@types/ejs": "3.1.5",
     "@types/node": "24.19.2",
     "acorn": "8.19.0",
diff --git a/public/css/auth.css b/public/css/auth.css
index b8fbe20..45fda1d 100644
--- a/public/css/auth.css
+++ b/public/css/auth.css
@@ -1,9 +1,11 @@
 .auth-stage {
-  min-height: 100dvh;
   display: grid; place-items: center;
   padding: 40px 20px 96px;
   background: var(--bg);
 }
+@supports (min-height: 100dvh) {
+  .auth-stage { min-height: 100dvh; }
+}
 .auth {
   width: 100%; max-width: 408px;
   display: flex; flex-direction: column; gap: 22px;
@@ -103,8 +105,10 @@
 .input:focus { outline: none; border-color: var(--accent);
   box-shadow: 0 0 0 2px var(--accent-bg); }
 /* gentle validation feedback only after the user has interacted */
-.input:user-invalid { border-color: var(--neg); }
-.input:user-invalid:focus { box-shadow: 0 0 0 2px var(--neg-bg); }
+@supports selector(:user-invalid) {
+  .input:user-invalid { border-color: var(--neg); }
+  .input:user-invalid:focus { box-shadow: 0 0 0 2px var(--neg-bg); }
+}
 
 .field-hint { font-size: var(--fz-xs); color: var(--text-faint); }
 
diff --git a/public/css/styles.css b/public/css/styles.css
index 92add70..7a14441 100644
--- a/public/css/styles.css
+++ b/public/css/styles.css
@@ -71,9 +71,24 @@
   line-height: 1.45;
 }
 
-/* shared DARK mapping, applied to Auto-via-OS and forced-dark */
-@media (prefers-color-scheme: dark) {
-  :root:not(:has(#theme-light:checked)):not(:has(#theme-dark:checked)) {
+@supports selector(:has(a)) {
+  /* shared DARK mapping, applied to Auto-via-OS and forced-dark */
+  @media (prefers-color-scheme: dark) {
+    :root:not(:has(#theme-light:checked)):not(:has(#theme-dark:checked)) {
+      --bg:var(--d-bg); --surface:var(--d-surface); --surface-2:var(--d-surface-2); --surface-3:var(--d-surface-3);
+      --border:var(--d-border); --border-2:var(--d-border-2);
+      --text:var(--d-text); --text-muted:var(--d-text-muted); --text-faint:var(--d-text-faint);
+      --accent:var(--d-accent); --accent-bg:var(--d-accent-bg); --accent-bd:var(--d-accent-bd); --focus:var(--d-focus); --on-accent:var(--d-on-accent);
+      --pos:var(--d-pos); --pos-bg:var(--d-pos-bg); --pos-bd:var(--d-pos-bd);
+      --neg:var(--d-neg); --neg-bg:var(--d-neg-bg); --neg-bd:var(--d-neg-bd);
+      --warn:var(--d-warn); --warn-bg:var(--d-warn-bg); --warn-bd:var(--d-warn-bd);
+      --info:var(--d-info); --info-bg:var(--d-info-bg); --info-bd:var(--d-info-bd);
+      color-scheme: dark;
+    }
+  }
+
+  /* forced DARK (overrides everything) */
+  html:has(#theme-dark:checked) {
     --bg:var(--d-bg); --surface:var(--d-surface); --surface-2:var(--d-surface-2); --surface-3:var(--d-surface-3);
     --border:var(--d-border); --border-2:var(--d-border-2);
     --text:var(--d-text); --text-muted:var(--d-text-muted); --text-faint:var(--d-text-faint);
@@ -84,32 +99,19 @@
     --info:var(--d-info); --info-bg:var(--d-info-bg); --info-bd:var(--d-info-bd);
     color-scheme: dark;
   }
-}
 
-/* forced DARK (overrides everything) */
-html:has(#theme-dark:checked) {
-  --bg:var(--d-bg); --surface:var(--d-surface); --surface-2:var(--d-surface-2); --surface-3:var(--d-surface-3);
-  --border:var(--d-border); --border-2:var(--d-border-2);
-  --text:var(--d-text); --text-muted:var(--d-text-muted); --text-faint:var(--d-text-faint);
-  --accent:var(--d-accent); --accent-bg:var(--d-accent-bg); --accent-bd:var(--d-accent-bd); --focus:var(--d-focus); --on-accent:var(--d-on-accent);
-  --pos:var(--d-pos); --pos-bg:var(--d-pos-bg); --pos-bd:var(--d-pos-bd);
-  --neg:var(--d-neg); --neg-bg:var(--d-neg-bg); --neg-bd:var(--d-neg-bd);
-  --warn:var(--d-warn); --warn-bg:var(--d-warn-bg); --warn-bd:var(--d-warn-bd);
-  --info:var(--d-info); --info-bg:var(--d-info-bg); --info-bd:var(--d-info-bd);
-  color-scheme: dark;
-}
-
-/* forced LIGHT (overrides the OS-dark media query) */
-html:has(#theme-light:checked) {
-  --bg:var(--l-bg); --surface:var(--l-surface); --surface-2:var(--l-surface-2); --surface-3:var(--l-surface-3);
-  --border:var(--l-border); --border-2:var(--l-border-2);
-  --text:var(--l-text); --text-muted:var(--l-text-muted); --text-faint:var(--l-text-faint);
-  --accent:var(--l-accent); --accent-bg:var(--l-accent-bg); --accent-bd:var(--l-accent-bd); --focus:var(--l-focus); --on-accent:var(--l-on-accent);
-  --pos:var(--l-pos); --pos-bg:var(--l-pos-bg); --pos-bd:var(--l-pos-bd);
-  --neg:var(--l-neg); --neg-bg:var(--l-neg-bg); --neg-bd:var(--l-neg-bd);
-  --warn:var(--l-warn); --warn-bg:var(--l-warn-bg); --warn-bd:var(--l-warn-bd);
-  --info:var(--l-info); --info-bg:var(--l-info-bg); --info-bd:var(--l-info-bd);
-  color-scheme: light;
+  /* forced LIGHT (overrides the OS-dark media query) */
+  html:has(#theme-light:checked) {
+    --bg:var(--l-bg); --surface:var(--l-surface); --surface-2:var(--l-surface-2); --surface-3:var(--l-surface-3);
+    --border:var(--l-border); --border-2:var(--l-border-2);
+    --text:var(--l-text); --text-muted:var(--l-text-muted); --text-faint:var(--l-text-faint);
+    --accent:var(--l-accent); --accent-bg:var(--l-accent-bg); --accent-bd:var(--l-accent-bd); --focus:var(--l-focus); --on-accent:var(--l-on-accent);
+    --pos:var(--l-pos); --pos-bg:var(--l-pos-bg); --pos-bd:var(--l-pos-bd);
+    --neg:var(--l-neg); --neg-bg:var(--l-neg-bg); --neg-bd:var(--l-neg-bd);
+    --warn:var(--l-warn); --warn-bg:var(--l-warn-bg); --warn-bd:var(--l-warn-bd);
+    --info:var(--l-info); --info-bg:var(--l-info-bg); --info-bd:var(--l-info-bd);
+    color-scheme: light;
+  }
 }
 
 /* ---------- 2. RESET ---------------------------------------- */
@@ -122,10 +124,12 @@ a { color: inherit; text-decoration: none; }
 summary::-webkit-details-marker { display: none; }
 summary { list-style: none; cursor: pointer; }
 
-:focus-visible {
-  outline: 2px solid var(--focus);
-  outline-offset: 1px;
-  border-radius: 3px;
+@supports selector(:focus-visible) {
+  :focus-visible {
+    outline: 2px solid var(--focus);
+    outline-offset: 1px;
+    border-radius: 3px;
+  }
 }
 @media (prefers-reduced-motion: no-preference) {
   .sidebar, .scrim, summary, .nav-item, .btn, .chip, .profile, .kebab { transition: .15s ease; }
@@ -153,7 +157,9 @@ summary { list-style: none; cursor: pointer; }
 .app {
   display: grid;
   grid-template-columns: var(--nav-w) minmax(0, 1fr);
-  min-height: 100dvh;
+}
+@supports (min-height: 100dvh) {
+  .app { min-height: 100dvh; }
 }
 
 /* ---------- 4. SIDEBAR -------------------------------------- */
@@ -219,7 +225,10 @@ summary { list-style: none; cursor: pointer; }
   min-height: 30px; padding: 4px 8px;
   border-radius: var(--radius);
   color: var(--text-muted); font-size: var(--fz);
-  position: relative; user-select: none;
+  position: relative;
+}
+@supports (user-select: none) {
+  .nav-self { user-select: none; }
 }
 a.nav-self:hover { background: var(--surface-2); color: var(--text); }
 .nav-self .ico { color: var(--text-faint); }
@@ -277,12 +286,14 @@ span.nav-self { cursor: default; }                 /* static / non-clickable */
 }
 .theme-switch label:hover { color: var(--text); }
 .theme-switch input { position: absolute; opacity: 0; inset: 0; cursor: pointer; }
-.theme-switch label:has(input:checked) {
-  background: var(--surface); color: var(--text);
-  box-shadow: 0 1px 2px rgba(0,0,0,.06); font-weight: 550;
-}
-.theme-switch label:has(input:focus-visible) {
-  outline: 2px solid var(--focus); outline-offset: 1px;
+@supports selector(:has(:focus-visible)) {
+  .theme-switch label:has(input:checked) {
+    background: var(--surface); color: var(--text);
+    box-shadow: 0 1px 2px rgba(0,0,0,.06); font-weight: 550;
+  }
+  .theme-switch label:has(input:focus-visible) {
+    outline: 2px solid var(--focus); outline-offset: 1px;
+  }
 }
 
 /* Without :has() every branch is open and every theme rule is inert: neither control does anything.
@@ -414,7 +425,7 @@ span.nav-self { cursor: default; }                 /* static / non-clickable */
   position: relative; display: inline-flex; align-items: center;
 }
 .select select {
-  appearance: none; height: 30px; padding: 0 26px 0 9px;
+  height: 30px; padding: 0 26px 0 9px;
   background: var(--surface); color: var(--text);
   border: 1px solid var(--border-2); border-radius: var(--radius);
   font: inherit; font-size: var(--fz-sm); cursor: pointer;
@@ -424,8 +435,13 @@ span.nav-self { cursor: default; }                 /* static / non-clickable */
   width: 7px; height: 7px; border-right: 1.5px solid var(--text-faint);
   border-bottom: 1.5px solid var(--text-faint); transform: translateY(-2px) rotate(45deg);
 }
-.select select:focus-visible { border-color: var(--accent); outline: none;
-  box-shadow: 0 0 0 2px var(--accent-bg); }
+@supports (appearance: none) {
+  .select select { appearance: none; }
+}
+@supports selector(:focus-visible) {
+  .select select:focus-visible { border-color: var(--accent); outline: none;
+    box-shadow: 0 0 0 2px var(--accent-bg); }
+}
 
 /* segmented toggle (radios) */
 .segmented {
@@ -440,11 +456,13 @@ span.nav-self { cursor: default; }                 /* static / non-clickable */
 }
 .segmented label:hover { color: var(--text); }
 .segmented input { position: absolute; opacity: 0; inset: 0; cursor: pointer; }
-.segmented label:has(input:checked) {
-  background: var(--surface); color: var(--text); font-weight: 550;
-  box-shadow: 0 1px 2px rgba(0,0,0,.06);
+@supports selector(:has(:focus-visible)) {
+  .segmented label:has(input:checked) {
+    background: var(--surface); color: var(--text); font-weight: 550;
+    box-shadow: 0 1px 2px rgba(0,0,0,.06);
+  }
+  .segmented label:has(input:focus-visible) { outline: 2px solid var(--focus); outline-offset: 1px; }
 }
-.segmented label:has(input:focus-visible) { outline: 2px solid var(--focus); outline-offset: 1px; }
 .seg-count { font-size: var(--fz-xs); color: var(--text-faint); }
 
 /* chips (multi-select checkboxes) */
@@ -460,12 +478,14 @@ span.nav-self { cursor: default; }                 /* static / non-clickable */
 .chip input { position: absolute; opacity: 0; inset: 0; cursor: pointer; }
 .chip .chip-dot { width: 7px; height: 7px; border-radius: 50%;
   background: var(--border-2); }
-.chip:has(input:checked) {
-  background: var(--accent-bg); border-color: var(--accent-bd);
-  color: var(--text); font-weight: 550;
+@supports selector(:has(:focus-visible)) {
+  .chip:has(input:checked) {
+    background: var(--accent-bg); border-color: var(--accent-bd);
+    color: var(--text); font-weight: 550;
+  }
+  .chip:has(input:checked) .chip-dot { background: var(--accent); }
+  .chip:has(input:focus-visible) { outline: 2px solid var(--focus); outline-offset: 1px; }
 }
-.chip:has(input:checked) .chip-dot { background: var(--accent); }
-.chip:has(input:focus-visible) { outline: 2px solid var(--focus); outline-offset: 1px; }
 
 /* date range */
 .daterange {
@@ -486,8 +506,10 @@ span.nav-self { cursor: default; }                 /* static / non-clickable */
   display: inline-flex; align-items: center; gap: 7px;
   font-size: var(--fz-sm); color: var(--text-muted); cursor: pointer;
 }
-.check input, .radio input { width: 15px; height: 15px; accent-color: var(--accent);
-  margin: 0; cursor: pointer; }
+.check input, .radio input { width: 15px; height: 15px; margin: 0; cursor: pointer; }
+@supports (accent-color: auto) {
+  .check input, .radio input { accent-color: var(--accent); }
+}
 .check:hover, .radio:hover { color: var(--text); }
 /* A stacked group of .check rows in a <fieldset> — the right element for related checkboxes, but the
    UA gives it a groove border, so reset it like .filter-field and .menu-field do. A disabled row is
@@ -495,7 +517,9 @@ span.nav-self { cursor: default; }                 /* static / non-clickable */
 .check-group { border: 0; margin: 0; padding: 0; display: flex; flex-direction: column; gap: 10px; }
 .check-group .check { align-items: baseline; }
 .check-group .check input:disabled { cursor: default; }
-.check-group .check:has(input:disabled) { opacity: .7; cursor: default; }
+@supports selector(:has(a)) {
+  .check-group .check:has(input:disabled) { opacity: .7; cursor: default; }
+}
 .check-group .check .cell-muted { margin-left: auto; padding-left: 12px; font-size: var(--fz-xs); }
 
 /* popover menu (language picker, profile, row kebab) — a <button popovertarget> and its [popover]
@@ -506,17 +530,25 @@ span.nav-self { cursor: default; }                 /* static / non-clickable */
 .menu { display: inline-flex; }
 .menu-pop {
   position: absolute; inset: auto; margin: 6px 0 0;
-  position-anchor: auto;
-  position-try-fallbacks: flip-block, flip-inline;
-  top: anchor(bottom); right: anchor(right);
   min-width: 210px; max-width: min(320px, 92vw); padding: 6px;
-  max-height: 60vh; overflow-y: auto; overflow-wrap: anywhere;
+  max-height: 60vh; overflow-y: auto;
   background: var(--surface); color: var(--text);
   border: 1px solid var(--border-2); border-radius: var(--radius);
   box-shadow: 0 8px 28px rgba(0,0,0,.16);
 }
-.menu-pop.left { right: auto; left: anchor(left); }
-.menu-pop.up { top: auto; bottom: anchor(top); margin: 0 0 6px; }
+.menu-pop.up { margin: 0 0 6px; }
+@supports (overflow-wrap: anywhere) {
+  .menu-pop { overflow-wrap: anywhere; }
+}
+@supports (position-anchor: auto) and (position-try-fallbacks: flip-block, flip-inline) and (top: anchor(bottom)) {
+  .menu-pop {
+    position-anchor: auto;
+    position-try-fallbacks: flip-block, flip-inline;
+    top: anchor(bottom); right: anchor(right);
+  }
+  .menu-pop.left { right: auto; left: anchor(left); }
+  .menu-pop.up { top: auto; bottom: anchor(top); }
+}
 /* A browser too old for the Popover API leaves the trigger inert, so let each panel flow under its
    trigger rather than stranding Sign out and the language picker behind a button that does nothing.
    The wrapper turns block so the pair stacks instead of sitting side by side, and min-width goes —
@@ -547,7 +579,10 @@ span.nav-self { cursor: default; }                 /* static / non-clickable */
 .menu-check { display: flex; align-items: center; gap: 9px; width: 100%;
   padding: 6px 8px; border-radius: 4px; font-size: var(--fz-sm); cursor: pointer; }
 .menu-check:hover { background: var(--surface-2); }
-.menu-check input { accent-color: var(--accent); width: 15px; height: 15px; margin: 0; }
+.menu-check input { width: 15px; height: 15px; margin: 0; }
+@supports (accent-color: auto) {
+  .menu-check input { accent-color: var(--accent); }
+}
 
 /* active filter pills */
 .active-pills { display: flex; align-items: center; gap: 7px; flex-wrap: wrap; }
@@ -589,11 +624,15 @@ table.table {
 }
 .table tbody th { font-weight: inherit; text-align: left; }   /* row-header cell, not bold/centered */
 .table tbody tr:hover td, .table tbody tr:hover th { background: var(--surface-2); }
-.table tbody tr:has(.row-select:checked) td,
-.table tbody tr:has(.row-select:checked) th { background: var(--accent-bg); }
+@supports selector(:has(a)) {
+  .table tbody tr:has(.row-select:checked) td,
+  .table tbody tr:has(.row-select:checked) th { background: var(--accent-bg); }
+}
 .col-check { width: 36px; text-align: center; padding: 0; }
-.col-check input { accent-color: var(--accent); width: 15px; height: 15px;
-  margin: 0; vertical-align: middle; }
+.col-check input { width: 15px; height: 15px; margin: 0; vertical-align: middle; }
+@supports (accent-color: auto) {
+  .col-check input { accent-color: var(--accent); }
+}
 .col-num { text-align: right; }
 .cell-strong { font-weight: 550; }
 .cell-muted { color: var(--text-muted); }
@@ -655,7 +694,9 @@ th[aria-sort="descending"] .sort-ico { transform: rotate(180deg); }
 .kebab:hover { background: var(--surface-2); color: var(--text); }
 /* Held open — a trigger's panel is its next sibling. Its own rule: :has() is non-forgiving, so an
    engine that doesn't know :popover-open would drop the hover style along with it. */
-.kebab:has(+ .menu-pop:popover-open) { background: var(--surface-2); color: var(--text); }
+@supports selector(.kebab:has(+ .menu-pop:popover-open)) {
+  .kebab:has(+ .menu-pop:popover-open) { background: var(--surface-2); color: var(--text); }
+}
 
 /* ---------- 10. PAGINATION --------------------------------- */
 .pager {
diff --git a/src/ui/stylesheet-floor.test.ts b/src/ui/stylesheet-floor.test.ts
index e37bfa0..d4f1aa1 100644
--- a/src/ui/stylesheet-floor.test.ts
+++ b/src/ui/stylesheet-floor.test.ts
@@ -1,16 +1,71 @@
 import assert from "node:assert/strict";
-import { readdirSync, readFileSync } from "node:fs";
+import { globSync, readFileSync } from "node:fs";
 import { dirname, join } from "node:path";
 import { test } from "node:test";
 import { fileURLToPath } from "node:url";
+import { floorViolations } from "./stylesheet-floor.ts";
 
-const cssDir = join(dirname(fileURLToPath(import.meta.url)), "..", "..", "public", "css");
+const root = join(dirname(fileURLToPath(import.meta.url)), "..", "..");
+const cssDir = join(root, "public", "css");
 
-// A custom property holding oklch() passes unchecked on an engine below Chromium 111, so the
-// var() that reads it is invalid at computed-value time: white text on no background.
-for (const file of readdirSync(cssDir).filter((f) => f.endsWith(".css"))) {
-  test(`${file} declares no oklch() colour`, () => {
-    assert.ok(!/oklch\(/i.test(readFileSync(join(cssDir, file), "utf8")));
+test("floorViolations flags a feature the floor lacks, with its line and the first version that has it", () => {
+  assert.deepEqual(floorViolations(".a { color: red; }\n.b:has(.c) { color: red; }"), [
+    "line 2: :has needs chrome 105, safari 15.4, firefox 121 (css.selectors.has)",
+  ]);
+  assert.deepEqual(floorViolations(".a { min-height: 100dvh; }"), [
+    "line 1: dvh needs chrome 108, safari 15.4 (css.types.length.viewport_percentage_units_dynamic)",
+  ]);
+  assert.deepEqual(floorViolations(".a { accent-color: red; }"), [
+    "line 1: accent-color needs chrome 93, safari 15.4 (css.properties.accent-color)",
+  ]);
+  assert.deepEqual(floorViolations(".a { overflow-wrap: anywhere; }"), [
+    "line 1: overflow-wrap: anywhere needs safari 15.4 (css.properties.overflow-wrap.anywhere)",
+  ]);
+  assert.equal(floorViolations(".a { & .b { color: red; } }").length, 1);
+});
+
+test("floorViolations checks a custom property's value, since the var() reading it fails unseen", () => {
+  assert.deepEqual(floorViolations(":root { --accent: oklch(0.5 0.1 250); }"), [
+    "line 1: oklch() needs chrome 111, safari 15.4 (css.types.color.oklch)",
+  ]);
+});
+
+test("floorViolations allows a feature inside an @supports that tests it, and only then", () => {
+  assert.deepEqual(floorViolations("@supports selector(:has(a)) { .b:has(.c) { color: red; } }"), []);
+  assert.deepEqual(floorViolations("@media (min-width: 1px) { @supports (min-height: 100dvh) { .a { min-height: 100dvh; } } }"), []);
+  assert.equal(floorViolations("@supports not selector(:has(a)) { .b:has(.c) { color: red; } }").length, 1);
+  assert.equal(floorViolations("@supports (display: grid) and (not selector(:has(a))) { .b:has(.c) { color: red; } }").length, 1);
+  assert.equal(floorViolations("@supports (min-height: 100dvh) { .b:has(.c) { color: red; } }").length, 1);
+  assert.equal(floorViolations("@supports (position-anchor: auto) { .a { top: anchor(bottom); } }").length, 1);
+});
+
+test("floorViolations passes what the floor has, a vendor prefix and a comment", () => {
+  const css = `/* .x:has(y) { min-height: 100dvh } */
+    @media (prefers-color-scheme: dark) { :root { color-scheme: dark; } }
+    .a::before, .a:focus-within { content: "{ :has( }"; display: grid; gap: 4px; inset: 0;
+      grid-template-columns: repeat(3, minmax(0, 1fr)); width: min(86vw, 20rem); transition: .15s ease;
+      background: rgba(0,0,0,.4) url("data:image/svg+xml;utf8,<svg/>"); transform: rotate(45deg); }
+    .b { -webkit-font-smoothing: antialiased; -webkit-user-select: none; }
+    summary::-webkit-details-marker { display: none; }`;
+  assert.deepEqual(floorViolations(css), []);
+});
+
+test("floorViolations refuses what MDN has no entry for, so a typo or a new feature can't pass unchecked", () => {
+  assert.deepEqual(floorViolations(".a { colr: red; }"), ["line 1: colr has no MDN compat entry"]);
+  assert.equal(floorViolations(".a { width: 3zz; }").length, 1);
+  assert.equal(floorViolations(".a:hovr { color: red; }").length, 1);
+});
+
+const stylesheets = globSync(["public/**/*.css", "examples/**/*.css"], { cwd: root, exclude: ["**/node_modules/**"] });
+
+test("the floor check finds the stylesheets", () => {
+  assert.ok(stylesheets.includes("public/css/styles.css"));
+  assert.ok(stylesheets.includes("examples/plugins/scheduling/public/scheduling.css"));
+});
+
+for (const file of stylesheets) {
+  test(`${file} uses nothing the browser floor lacks outside an @supports guarding it`, () => {
+    assert.deepEqual(floorViolations(readFileSync(join(root, file), "utf8")), []);
   });
 }
 
@@ -31,8 +86,8 @@ function blockEnd(start: number): number {
 // Chromium below 105 and Firefox below 121 have no `:has()`, and nothing else reveals a folded
 // branch there, so the fold may only exist where `:has()` works.
 test("styles.css folds .nav-children only inside @supports selector(:has(a))", () => {
-  const open = stylesheet.indexOf("@supports selector(:has(a)) {");
-  assert.ok(open >= 0, "no @supports selector(:has(a)) block");
+  const open = stylesheet.lastIndexOf("@supports selector(:has(a)) {", stylesheet.indexOf(".nav-node > .nav-children"));
+  assert.ok(open >= 0, "no @supports selector(:has(a)) block before the fold rule");
   const close = blockEnd(open);
   const outside = stylesheet.slice(0, open) + stylesheet.slice(close + 1);
   assert.ok(!/\.nav-children\s*\{[^}]*display:\s*none/.test(outside), "a .nav-children rule hides it outside the guard");
diff --git a/src/ui/stylesheet-floor.ts b/src/ui/stylesheet-floor.ts
new file mode 100644
index 0000000..03caaf5
--- /dev/null
+++ b/src/ui/stylesheet-floor.ts
@@ -0,0 +1,207 @@
+// README → "Two browser floors", checked against MDN's compat data: a feature one of FLOOR's engines
+// lacks passes only inside an `@supports` whose condition names it. Checked: at-rules, media features,
+// pseudo-classes and -elements, properties, and in a value its functions, its units and the keywords
+// MDN lists for that property. A partial implementation counts as support. A vendor-prefixed name is
+// one engine's by design and is skipped, as are `font-family` names: an engine skips a family it lacks.
+// Not checked: the body of an at-rule other than @media and @supports, and syntax MDN has no entry for,
+// such as media range syntax.
+import bcd from "@mdn/browser-compat-data" with { type: "json" };
+import type { CompatStatement, Identifier, SimpleSupportStatement } from "@mdn/browser-compat-data";
+
+const FLOOR = { chrome: "87", safari: "15", firefox: "115" };
+
+interface Feature {
+  label: string;
+  path: string | undefined;
+  // Matches the feature in an `@supports` condition that tests for it.
+  guard: RegExp;
+  offset: number;
+}
+
+interface Sheet {
+  css: string;
+  out: string[];
+}
+
+export function floorViolations(css: string): string[] {
+  const sheet = { css: blank(css), out: [] };
+  walk(sheet, 0, sheet.css.length, []);
+  return [...new Set(sheet.out)];
+}
+
+// Comments, string contents and url() contents become spaces, so nothing inside one reads as syntax
+// and every offset keeps its line.
+function blank(css: string): string {
+  return css.replace(/\/\*[\s\S]*?\*\/|"(?:[^"\\\n]|\\.)*"|'(?:[^'\\\n]|\\.)*'|url\([^)]*\)/g, (m) => {
+    if (m.startsWith("/*")) return m.replace(/[^\n]/g, " ");
+    const keep = m.startsWith("url(") ? 4 : 1;
+    return m.slice(0, keep) + " ".repeat(m.length - keep - 1) + m.slice(-1);
+  });
+}
+
+function closing(text: string, open: number): number {
+  const [opener, closer] = text[open] === "(" ? ["(", ")"] : ["{", "}"];
+  let depth = 0;
+  for (let i = open; i < text.length; i++) {
+    if (text[i] === opener) depth++;
+    if (text[i] === closer && --depth === 0) return i;
+  }
+  throw new Error(`unbalanced ${opener} at offset ${open}`);
+}
+
+function walk(sheet: Sheet, start: number, end: number, guards: string[]): void {
+  let i = start;
+  while (i < end) {
+    const stop = sheet.css.slice(i, end).search(/[{;]/);
+    if (stop < 0) return;
+    const at = i + stop;
+    const raw = sheet.css.slice(i, at);
+    const prelude = raw.trim();
+    const offset = i + raw.length - raw.trimStart().length;
+    const blockEnd = sheet.css[at] === "{" ? closing(sheet.css, at) : at;
+    if (prelude.startsWith("@")) atRule(sheet, prelude, offset, guards, [at + 1, blockEnd]);
+    else styleRule(sheet, prelude, offset, guards, [at + 1, blockEnd]);
+    i = blockEnd + 1;
+  }
+}
+
+function atRule(sheet: Sheet, prelude: string, offset: number, guards: string[], [from, to]: [number, number]): void {
+  const name = /^@([\w-]+)/.exec(prelude)?.[1]?.toLowerCase() ?? "";
+  if (name.startsWith("-")) return;
+  check(sheet, guards, { label: `@${name}`, path: `css.at-rules.${name}`, guard: never, offset });
+  const condition = prelude.slice(name.length + 1);
+  if (name === "media") {
+    for (const m of condition.matchAll(/\(\s*(?:min-|max-)?([a-z][\w-]*)\s*[:)]/gi)) {
+      check(sheet, guards, { label: `(${m[1]})`, path: `css.at-rules.media.${m[1]}`, guard: never, offset });
+    }
+  }
+  if (name === "supports" && /(?<![\w-])selector\(/i.test(condition)) {
+    check(sheet, guards, { label: "selector()", path: "css.at-rules.supports.selector", guard: never, offset });
+  }
+  if (name === "media" || name === "supports") {
+    walk(sheet, from, to, name === "supports" ? [...guards, positive(condition)] : guards);
+  }
+}
+
+// The parts of an @supports condition that hold inside it: a `not (…)` group holds only outside.
+function positive(condition: string): string {
+  let rest = condition;
+  for (let at = rest.search(/(?<![\w-])not\s/i); at >= 0; at = rest.search(/(?<![\w-])not\s/i)) {
+    const open = rest.indexOf("(", at);
+    if (open < 0) return rest.slice(0, at);
+    rest = rest.slice(0, at) + rest.slice(closing(rest, open) + 1);
+  }
+  return rest;
+}
+
+function styleRule(sheet: Sheet, selector: string, offset: number, guards: string[], [from, to]: [number, number]): void {
+  for (const m of selector.matchAll(/::?([a-z][\w-]*)/gi)) {
+    const name = (m[1] ?? "").toLowerCase();
+    check(sheet, guards, { label: `:${name}`, path: `css.selectors.${name}`, guard: new RegExp(`:${name}(?![\\w-])`, "i"), offset: offset + (m.index ?? 0) });
+  }
+  const body = sheet.css.slice(from, to);
+  if (body.includes("{")) {
+    check(sheet, guards, { label: "a nested rule", path: "css.selectors.nesting", guard: never, offset: from });
+    return;
+  }
+  let at = from;
+  for (const declaration of body.split(";")) {
+    const colon = declaration.indexOf(":");
+    if (colon > 0) property(sheet, guards, declaration.slice(0, colon).trim().toLowerCase(), declaration.slice(colon + 1), at + declaration.search(/\S/));
+    at += declaration.length + 1;
+  }
+}
+
+function property(sheet: Sheet, guards: string[], name: string, value: string, offset: number): void {
+  if (name.startsWith("-") && !name.startsWith("--")) return;
+  const custom = name.startsWith("--");
+  const known = custom || check(sheet, guards, { label: name, path: `css.properties.${name}`, guard: word(name), offset });
+  for (const m of value.matchAll(/(?<![\w-])([a-z][\w-]*)\(/gi)) {
+    const fn = m[1] ?? "";
+    check(sheet, guards, { label: `${fn}()`, path: functionPath(name, fn), guard: new RegExp(`(?<![\\w-])${fn}\\(`, "i"), offset });
+  }
+  for (const m of value.replace(/#[\da-f]+/gi, "").matchAll(/(?<![\w.])\d*\.?\d+([a-z]+|%)/gi)) {
+    const unit = (m[1] ?? "").toLowerCase();
+    check(sheet, guards, { label: unit, path: unitPath(unit), guard: new RegExp(`\\d${unit}(?![\\w-])`, "i"), offset });
+  }
+  if (custom || !known || name === "font-family") return;
+  for (const m of value.matchAll(/(?<![\w-])([a-z][\w-]*)(?![\w(-])/gi)) {
+    const path = `css.properties.${name}.${(m[1] ?? "").toLowerCase()}`;
+    if (entry(path)) check(sheet, guards, { label: `${name}: ${m[1]}`, path, guard: word(m[1] ?? ""), offset });
+  }
+}
+
+const never = /(?!)/;
+const word = (name: string): RegExp => new RegExp(`(?<![\\w-])${name}(?![\\w-])`, "i");
+
+// A name two types share, like rect() in <shape> and <basic-shape>, passes where the floor has either.
+function functionPath(property: string, fn: string): string | undefined {
+  const name = fn === "rgba" ? "rgb" : fn; // MDN files rgba() under rgb(), its alias
+  const types = Object.keys(bcd.css.types ?? {}).map((type) => `css.types.${type}.${name}`);
+  const paths = [`css.properties.${property}.${name}`, `css.types.${name}`, ...types].filter((path) => entry(path));
+  return paths.find((path) => lacking(entry(path)).length === 0) ?? paths[0];
+}
+
+const VIEWPORT: Record<string, string> = { d: "dynamic", s: "small", l: "large" };
+const UNIT_TYPES: Record<string, string> = {
+  px: "css.types.length",
+  "%": "css.types.percentage",
+  fr: "css.types.flex",
+  s: "css.types.time",
+  ms: "css.types.time",
+};
+
+function unitPath(unit: string): string | undefined {
+  const viewport = /^([dsl])v(?:h|w|i|b|min|max)$/.exec(unit)?.[1];
+  if (viewport) return `css.types.length.viewport_percentage_units_${VIEWPORT[viewport]}`;
+  return UNIT_TYPES[unit] ?? ["length", "angle", "resolution"].map((type) => `css.types.${type}.${unit}`).find((path) => entry(path));
+}
+
+function entry(path: string | undefined): CompatStatement | undefined {
+  if (!path) return undefined;
+  let node: Identifier | undefined = bcd as unknown as Identifier;
+  for (const key of path.split(".")) node = node && Object.hasOwn(node, key) ? node[key] : undefined;
+  return node?.__compat;
+}
+
+// Reports the feature unless a guard names it; true when it is fine as written.
+function check(sheet: Sheet, guards: string[], feature: Feature): boolean {
+  if (guards.some((condition) => feature.guard.test(condition))) return true;
+  const line = sheet.css.slice(0, feature.offset).split("\n").length;
+  const compat = entry(feature.path);
+  if (!compat) {
+    sheet.out.push(`line ${line}: ${feature.label} has no MDN compat entry`);
+    return false;
+  }
+  const missing = lacking(compat);
+  if (missing.length > 0) sheet.out.push(`line ${line}: ${feature.label} needs ${missing.join(", ")} (${feature.path})`);
+  return missing.length === 0;
+}
+
+// Each floor engine without the feature, with the release that brought it, as "safari 15.4".
+function lacking(compat: CompatStatement | undefined): string[] {
+  return Object.entries(FLOOR).flatMap(([browser, floor]) => {
+    const support = compat?.support[browser as keyof typeof compat.support];
+    const statements = [support ?? []].flat().filter((s: SimpleSupportStatement) => !s.prefix && !s.alternative_name && !s.flags);
+    const has = (s: SimpleSupportStatement): boolean => {
+      const added = release(s.version_added);
+      return added !== undefined && !newer(added, floor) && !(s.version_removed && !newer(s.version_removed, floor));
+    };
+    if (statements.some(has)) return [];
+    const releases = statements.flatMap((s) => release(s.version_added) ?? []).sort((x, y) => (newer(x, y) ? 1 : -1));
+    return [`${browser} ${releases[0] ?? "(none yet)"}`];
+  });
+}
+
+// A shipped release number; "≤79" counts as 79, "preview" and false as none.
+function release(version: string | false): string | undefined {
+  return typeof version === "string" && /^≤?\d/.test(version) ? version.replace("≤", "") : undefined;
+}
+
+function newer(version: string, than: string): boolean {
+  const [a, b] = [version, than].map((v) => v.split(".").map(Number));
+  for (let i = 0; i < Math.max(a?.length ?? 0, b?.length ?? 0); i++) {
+    if ((a?.[i] ?? 0) !== (b?.[i] ?? 0)) return (a?.[i] ?? 0) > (b?.[i] ?? 0);
+  }
+  return false;
+}
diff --git a/todo.md b/todo.md
index 10c04d5..2dec606 100644
--- a/todo.md
+++ b/todo.md
@@ -12,7 +12,6 @@
 
 | ID | Release | Blocked by | Exempt | Item | L | E | I | R | S | Moves | Score |
 |---|---|---|---|---|---|---|---|---|---|---|---|
-| 32 | 0.4.21 |  |  | **Fail the gate on a stylesheet feature the browser floor lacks, unless it is guarded.** | 4 | 2 | 2 | 6 | 7 | code health by 2 | 1.1 |
 | 13 | 0.4.21 |  |  | **Accept a verification code pasted with surrounding whitespace.** | 4 | 2 | 3 | 7 | 7 | 2 by 8 | 1.0 |
 | 19 | 0.5.0 |  | question | **Run the documented Docker commands on macOS and fix whatever misbehaves.** | 2 | 2 | 1 | 3 | 3 | 1 by 3 | 3.3 |
 | 16 | 0.5.0 |  | question | **Decide whether the single Keto `Resource` namespace splits into per-domain namespaces.** | 3 | 2 | 3 | 6 | 6 | 7 by 2 | 0.5 |
@@ -27,12 +26,6 @@
 
 ## Details
 
-### 32. Fail the gate on a stylesheet feature the browser floor lacks, unless it is guarded.
-
-Nothing checks README → "Two browser floors", and no E2E engine is old enough to show a break.
-Lint every stylesheet, the examples' included, against Chromium 87, Safari 15 and Firefox 115, and
-allow a feature only inside an `@supports` that guards it.
-
 ### 13. Accept a verification code pasted with surrounding whitespace.
 
 A pasted code fails the browser's `pattern` check. No zero-JS fix exists: `pattern`