diff --git a/packages/lint/src/rules/core.test.ts b/packages/lint/src/rules/core.test.ts
index 12502b48e1..1efc3865c9 100644
--- a/packages/lint/src/rules/core.test.ts
+++ b/packages/lint/src/rules/core.test.ts
@@ -870,6 +870,78 @@ describe("core rules", () => {
     });
   });
 
+  describe("runtime_hidden_style_opacity", () => {
+    const comp = (css: string, extraMarkup = "") => `
+<html><head><style>${css}</style></head><body>
+  <div id="root" data-composition-id="main" data-width="1920" data-height="1080">
+    <video id="footage" src="clip.mp4" data-start="0" data-duration="5" muted playsinline></video>
+    ${extraMarkup}
+  </div>
+  <script>window.__timelines = { main: gsap.timeline({ paused: true }) };</script>
+</body></html>`;
+
+    it("errors when a broad hidden-style selector forces replacement-frame opacity to zero", async () => {
+      const result = await lintHyperframeHtml(
+        comp(`[style*="visibility: hidden"] { opacity: 0 !important; }`),
+      );
+      const finding = result.findings.find((item) => item.code === "runtime_hidden_style_opacity");
+
+      expect(finding?.severity).toBe("error");
+      expect(finding?.selector).toBe(`[style*="visibility: hidden"]`);
+      expect(finding?.message).toContain("replacement frame");
+      expect(finding?.fixHint).toContain("data-composition-src");
+    });
+
+    it("errors when composition scoping still leaves the hidden-style selector on video", async () => {
+      const result = await lintHyperframeHtml(
+        comp(`#root > video[style*="visibility: hidden"] { opacity: 0; }`),
+      );
+
+      expect(
+        result.findings.find((item) => item.code === "runtime_hidden_style_opacity")?.selector,
+      ).toBe(`#root > video[style*="visibility: hidden"]`);
+    });
+
+    it("errors when the root stylesheet can affect video mounted from a sub-composition", async () => {
+      const result = await lintHyperframeHtml(`
+<html><head><style>[style*="visibility: hidden"] { opacity: 0; }</style></head><body>
+  <div id="root" data-composition-id="main" data-width="1920" data-height="1080">
+    <div data-composition-id="scene" data-composition-src="scene.html"></div>
+  </div>
+  <script>window.__timelines = { main: gsap.timeline({ paused: true }) };</script>
+</body></html>`);
+
+      expect(
+        result.findings.find((item) => item.code === "runtime_hidden_style_opacity")?.severity,
+      ).toBe("error");
+    });
+
+    it("allows hidden-style opacity guards scoped to sub-composition hosts", async () => {
+      const result = await lintHyperframeHtml(
+        comp(
+          `[data-composition-src][style*="visibility: hidden"],
+           [data-composition-file][style*="visibility: hidden"] { opacity: 0 !important; }`,
+          `<div data-composition-id="scene-a" data-composition-src="scene-a.html"></div>
+           <div data-composition-id="scene-b" data-composition-file="scene-b.html"></div>`,
+        ),
+      );
+
+      expect(
+        result.findings.find((item) => item.code === "runtime_hidden_style_opacity"),
+      ).toBeUndefined();
+    });
+
+    it("allows broad hidden-style selectors that do not change opacity", async () => {
+      const result = await lintHyperframeHtml(
+        comp(`[style*="visibility: hidden"] { pointer-events: none; }`),
+      );
+
+      expect(
+        result.findings.find((item) => item.code === "runtime_hidden_style_opacity"),
+      ).toBeUndefined();
+    });
+  });
+
   describe("css_parse_error — malformed CSS is reported instead of silently swallowed", () => {
     it("reports a css_parse_error finding for unparseable CSS", async () => {
       const html = `<html><body>
diff --git a/packages/lint/src/rules/core.ts b/packages/lint/src/rules/core.ts
index 6f8569034b..42c69335d9 100644
--- a/packages/lint/src/rules/core.ts
+++ b/packages/lint/src/rules/core.ts
@@ -97,6 +97,53 @@ function resolvedRuleSelectors(rule: postcss.Rule): string[] {
   );
 }
 
+function selectorAliasesRuntimeHiddenStyle(selector: string): boolean {
+  let unsafe = false;
+  try {
+    selectorParser((root) => {
+      root.each((selectorNode) => {
+        const subject: selectorParser.Node[] = [];
+        selectorNode.each((node) => {
+          if (node.type === "combinator") subject.length = 0;
+          else subject.push(node);
+        });
+
+        const hostScoped = subject.some(
+          (node) =>
+            node.type === "attribute" &&
+            ["data-composition-src", "data-composition-file"].includes(
+              node.attribute.toLowerCase(),
+            ),
+        );
+        if (hostScoped) return;
+
+        if (
+          subject.some((node) => {
+            if (node.type !== "attribute" || node.attribute.toLowerCase() !== "style") return false;
+            if (node.operator !== "*=" || !node.value) return false;
+            const needle = node.insensitive ? node.value.toLowerCase() : node.value;
+            if (!needle.includes("visibility") && !needle.includes("hidden")) return false;
+            return "visibility: hidden !important;".includes(needle);
+          })
+        ) {
+          unsafe = true;
+        }
+      });
+    }).processSync(selector);
+  } catch {
+    return false;
+  }
+  return unsafe;
+}
+
+function ruleForcesOpacityZero(rule: postcss.Rule): boolean {
+  let forcesOpacityZero = false;
+  rule.walkDecls(/^opacity$/i, (declaration) => {
+    if (Number(declaration.value.trim()) === 0) forcesOpacityZero = true;
+  });
+  return forcesOpacityZero;
+}
+
 function isStudioTimelineElement(tag: { raw: string; name: string }): boolean {
   if (["script", "style", "link", "meta", "template", "noscript"].includes(tag.name)) {
     return false;
@@ -333,10 +380,11 @@ export const coreRules: Array<(ctx: LintContext) => HyperframeLintFinding[]> = [
     return findings;
   },
 
-  // repeated_id_descendant_selector
+  // CSS selector safety
   ({ styles }) => {
     const findings: HyperframeLintFinding[] = [];
-    const reported = new Set<string>();
+    const reportedRepeatedIds = new Set<string>();
+    const reportedHiddenStyleSelectors = new Set<string>();
     for (const style of styles) {
       let root: postcss.Root;
       try {
@@ -350,16 +398,36 @@ export const coreRules: Array<(ctx: LintContext) => HyperframeLintFinding[]> = [
         continue;
       }
       root.walkRules((rule) => {
+        const forcesOpacityZero = ruleForcesOpacityZero(rule);
         for (const selector of resolvedRuleSelectors(rule)) {
           const repeatedId = repeatedDescendantId(selector);
-          if (!repeatedId || reported.has(repeatedId)) continue;
-          reported.add(repeatedId);
+          if (repeatedId && !reportedRepeatedIds.has(repeatedId)) {
+            reportedRepeatedIds.add(repeatedId);
+            findings.push({
+              code: "repeated_id_descendant_selector",
+              severity: "error",
+              message: `Selector "${selector}" requires #${repeatedId} to be nested inside another #${repeatedId}. IDs must be unique, so this selector cannot match a valid composition.`,
+              selector,
+              fixHint: `Remove the duplicate ancestor: change \`#${repeatedId} #${repeatedId}\` to \`#${repeatedId}\`.`,
+            });
+          }
+
+          if (
+            !forcesOpacityZero ||
+            reportedHiddenStyleSelectors.has(selector) ||
+            !selectorAliasesRuntimeHiddenStyle(selector)
+          ) {
+            continue;
+          }
+          reportedHiddenStyleSelectors.add(selector);
           findings.push({
-            code: "repeated_id_descendant_selector",
+            code: "runtime_hidden_style_opacity",
             severity: "error",
-            message: `Selector "${selector}" requires #${repeatedId} to be nested inside another #${repeatedId}. IDs must be unique, so this selector cannot match a valid composition.`,
+            message: `Selector "${selector}" observes HyperFrames' runtime-owned hidden style and forces opacity to zero. The renderer hides each native video before copying its computed opacity to the visible replacement frame, so this rule makes both transparent.`,
             selector,
-            fixHint: `Remove the duplicate ancestor: change \`#${repeatedId} #${repeatedId}\` to \`#${repeatedId}\`.`,
+            fixHint:
+              'Restrict the guard to sub-composition hosts, for example `[data-composition-src][style*="visibility: hidden"]` and `[data-composition-file][style*="visibility: hidden"]`. Do not derive arbitrary element or media opacity from runtime-owned inline visibility.',
+            snippet: truncateSnippet(rule.toString()),
           });
         }
       });