diff --git a/packages/diffs/src/editor/editor.css b/packages/diffs/src/editor/editor.css
index a65a8bf78..c50364d95 100644
--- a/packages/diffs/src/editor/editor.css
+++ b/packages/diffs/src/editor/editor.css
@@ -275,8 +275,14 @@
   border: 0;
   pointer-events: auto;
 
+  /* `contrast-color()` returns black or white against the severity fill. A
+     theme can set that fill through `editorWarning.foreground` and the similar
+     tokens, so a fixed text color can fail.
+     Pass one argument only. Two arguments make the declaration invalid at
+     computed-value time: `color` then inherits the editor foreground and
+     ignores the `--diffs-marker-contrast` declaration above. */
   @supports (color: contrast-color(red)) {
-    color: contrast-color(var(--diffs-marker-bg), var(--diffs-bg));
+    color: contrast-color(var(--diffs-marker-bg));
   }
 
   [data-marker-message] {
diff --git a/packages/diffs/test/cssContrastColorArity.test.ts b/packages/diffs/test/cssContrastColorArity.test.ts
new file mode 100644
index 000000000..e42eb2993
--- /dev/null
+++ b/packages/diffs/test/cssContrastColorArity.test.ts
@@ -0,0 +1,83 @@
+import { describe, expect, test } from 'bun:test';
+import { readFileSync } from 'fs';
+import { resolve } from 'path';
+
+// CSS files that ship with the package. The tests below check every
+// `contrast-color()` call in them.
+const STYLESHEETS = ['../src/style.css', '../src/editor/editor.css'];
+
+// Splits a `contrast-color()` argument list into its top-level arguments. A
+// comma inside `var()`, `light-dark()`, or `color-mix()` does not split.
+function splitTopLevelArgs(args: string): string[] {
+  const parts: string[] = [];
+  let depth = 0;
+  let current = '';
+  for (const char of args) {
+    if (char === '(') {
+      depth += 1;
+    } else if (char === ')') {
+      depth -= 1;
+    } else if (char === ',' && depth === 0) {
+      parts.push(current.trim());
+      current = '';
+      continue;
+    }
+    current += char;
+  }
+  parts.push(current.trim());
+  return parts;
+}
+
+// Returns the argument list of every `contrast-color()` call in `css`. It counts
+// nested parens to find the closing paren of each call.
+function findContrastColorArgs(css: string): string[] {
+  const calls: string[] = [];
+  const marker = 'contrast-color(';
+  let index = css.indexOf(marker);
+  while (index !== -1) {
+    let depth = 1;
+    let cursor = index + marker.length;
+    while (cursor < css.length && depth > 0) {
+      if (css[cursor] === '(') {
+        depth += 1;
+      } else if (css[cursor] === ')') {
+        depth -= 1;
+      }
+      cursor += 1;
+    }
+    calls.push(css.slice(index + marker.length, cursor - 1));
+    index = css.indexOf(marker, cursor);
+  }
+  return calls;
+}
+
+// Regression test for white text on the yellow warning popover.
+// `contrast-color()` takes one argument. A call with two arguments still parses,
+// because `var()` defers the grammar check to substitution. The declaration is
+// then invalid at computed-value time, so `color` inherits its value and ignores
+// the `--diffs-marker-contrast` declaration. No browser test catches this:
+// Playwright bundles an old Chromium without `contrast-color()`, so it runs the
+// fallback branch only.
+describe('contrast-color() arity in shipped CSS', () => {
+  for (const stylesheet of STYLESHEETS) {
+    test(`${stylesheet} passes one argument per call`, () => {
+      const css = readFileSync(resolve(__dirname, stylesheet), 'utf-8');
+      for (const args of findContrastColorArgs(css)) {
+        expect(
+          splitTopLevelArgs(args),
+          `contrast-color(${args}) must take exactly one argument`
+        ).toHaveLength(1);
+      }
+    });
+  }
+
+  test('detects a two-argument call', () => {
+    expect(
+      splitTopLevelArgs(
+        findContrastColorArgs(
+          'a { color: contrast-color(var(--a), light-dark(#000, #fff)); }'
+        )[0]
+      )
+    ).toHaveLength(2);
+  });
+});
diff --git a/packages/diffs/test/e2e/markers.pw.ts b/packages/diffs/test/e2e/markers.pw.ts
index 417ba3078..c3f1c7caa 100644
--- a/packages/diffs/test/e2e/markers.pw.ts
+++ b/packages/diffs/test/e2e/markers.pw.ts
@@ -87,7 +87,9 @@ function openScrolledMarkerNearGutter(page: Page): Promise<{
 // contrast ratio between the popover's resolved text and background colors.
 // Guards the marker popover fallback in browsers without contrast-color(): the
 // severity fill is a theme editorX.foreground token, so its text candidate must
-// remain legible in both light and dark themes.
+// remain legible in both light and dark themes. Playwright bundles an old
+// Chromium without contrast-color(), so this test measures the fallback branch
+// only. cssContrastColorArity.test.ts covers the `@supports` branch.
 async function popoverContrast(
   page: Page,
   token: string