fix(cli): skip contrast on transparent backdrops

packages/cli/src/commands/contrast-audit.browser.js

@@ -361,17 +361,25 @@ window.__contrastAuditFinish = async function (imgBase64, time, candidates) {
     var stepY = Math.max(1, Math.floor((y1 - y0) / 6));
     var rr = [],
       gg = [],
-      bb = [];
+      bb = [],
+      aa = [];
     for (var y = y0; y <= y1; y += stepY) {
       for (var x = x0; x <= x1; x += stepX) {
         var idx = (y * w + x) * 4;
         rr.push(px[idx]);
         gg.push(px[idx + 1]);
         bb.push(px[idx + 2]);
+        aa.push(px[idx + 3]);
       }
     }
     if (rr.length === 0) continue;
 
+    // A transparent sampled backdrop is supplied by the downstream editor,
+    // not this composition. WCAG contrast cannot be inferred until that live
+    // video/image is known, so do not invent an opaque browser default and
+    // hard-fail an otherwise valid alpha overlay.
+    if (median(aa) < 255) continue;
+
     var bgR = median(rr),
       bgG = median(gg),
       bgB = median(bb);

packages/cli/src/commands/layout-audit.browser.test.ts

@@ -605,6 +605,36 @@ describe("contrast-audit.browser background sampling", () => {
     expect(result).toHaveLength(1);
     expect(result[0]).toMatchObject({ selector: "#label", wcagAA: true, bg: "rgb(10,10,10)" });
   });
+
+  it("skips text whose sampled backdrop remains transparent", async () => {
+    document.body.innerHTML = `
+      <div id="root" data-composition-id="overlay" data-width="640" data-height="360">
+        <span id="label">Live</span>
+      </div>
+    `;
+
+    vi.spyOn(window, "getComputedStyle").mockImplementation(
+      () =>
+        ({
+          display: "block",
+          visibility: "visible",
+          opacity: "1",
+          color: "rgb(255, 255, 255)",
+          fontSize: "20px",
+          fontWeight: "700",
+          clipPath: "none",
+        }) as unknown as CSSStyleDeclaration,
+    );
+    vi.spyOn(document.getElementById("label")!, "getBoundingClientRect").mockReturnValue(
+      rect({ left: 50, top: 50, width: 100, height: 30 }),
+    );
+    (document as unknown as { elementFromPoint: () => Element | null }).elementFromPoint = () =>
+      null;
+
+    installContrastScript(new Uint8ClampedArray(640 * 360 * 4));
+
+    expect(await runContrastAudit()).toEqual([]);
+  });
 });
 
 // Both blocks overlap heavily; only the exemption on block A should suppress

packages/cli/src/commands/validate.ts

@@ -276,7 +276,11 @@ async function runContrastAudit(page: import("puppeteer-core").Page): Promise<Co
           : [],
       )) as ContrastCandidate[];
 
-      const screenshot = (await page.screenshot({ encoding: "base64", type: "png" })) as string;
+      const screenshot = (await page.screenshot({
+        encoding: "base64",
+        type: "png",
+        omitBackground: true,
+      })) as string;
       const entries = await page.evaluate(
         (b64: string, time: number, cands: ContrastCandidate[]) =>
           typeof (window as unknown as Record<string, unknown>).__contrastAuditFinish === "function"

packages/cli/src/utils/checkBrowser.ts

@@ -510,7 +510,11 @@ async function collectContrast(
     // This screenshot is the one contrast math is sampled from below — it must
     // stay untouched by the annotation overlay (finishContrast reads real
     // painted pixels), so annotation only ever happens on a SECOND shot.
-    const measurementShot = await page.screenshot({ encoding: "base64", type: "png" });
+    const measurementShot = await page.screenshot({
+      encoding: "base64",
+      type: "png",
+      omitBackground: true,
+    });
     if (typeof measurementShot !== "string") throw new Error("Contrast screenshot was not base64");
     const raw = await finishContrast(
       page,