diff --git a/media/preview.css b/media/preview.css
index fa944b0..f706ff1 100644
--- a/media/preview.css
+++ b/media/preview.css
@@ -35,6 +35,14 @@ del { text-decoration: line-through; opacity: 0.7; }
 .cw-del-claude { background: rgba(188,140,255,0.13); text-decoration: line-through; text-decoration-color: #bc8cff; }
 .cw-del-none   { background: var(--vscode-diffEditor-removedTextBackground, rgba(248,81,73,0.11)); text-decoration: line-through; opacity: 0.7; }
 
+/* Task 8 — overlap: if a future render path nests cw-ins-{A} inside cw-del-{B},
+   both marks must show. cw-ins-* sets text-decoration:none which would otherwise
+   suppress the parent del's strikethrough. `inherit` restores the parent's
+   line-through while the child's border-bottom underline is unaffected.
+   The engine currently emits SIBLING ins/del (never nested) so this rule is a
+   forward defensive guarantee only. See task-8-report.md for details. */
+.cw-del-claude .cw-ins-human, .cw-del-human .cw-ins-claude { text-decoration: inherit; }
+
 /* whole-block ops: an added block is an insertion (its author runs are emitted as
    cw-ins-* via colorByAuthor with kind="ins"); a standalone removed block is neutral
    struck (adjacency heuristic fallback) */
diff --git a/test/trackChangesModel.test.ts b/test/trackChangesModel.test.ts
index 9e350a0..101ea91 100644
--- a/test/trackChangesModel.test.ts
+++ b/test/trackChangesModel.test.ts
@@ -246,6 +246,21 @@ describe("wordDiffByAuthor", () => {
     expect(html).not.toContain("cw-del-human");
     expect(html).not.toContain("cw-del-claude");
   });
+  test("emits SIBLING ins/del elements, never nested — CSS overlap rule in preview.css is a defensive forward guarantee", () => {
+    // The engine assigns the cluster's insertion author to all deletions in the same
+    // cluster (adjacency heuristic), so <del> and <ins> always carry the SAME author
+    // as siblings. Cross-author nesting (<del class="cw-del-X"><ins class="cw-ins-Y">)
+    // is never produced today. The CSS rule `.cw-del-claude .cw-ins-human { text-decoration: inherit }`
+    // guards against a future regression where such nesting appears.
+    const spans: AuthorSpan[] = [{ start: 0, end: 100, author: "human" }];
+    const html = wordDiffByAuthor("old text", "new text", 0, spans);
+    expect(html).toContain('<del class="cw-del-human">');
+    expect(html).toContain('<ins class="cw-ins-human">');
+    // Structural check: no <del> immediately wraps an <ins> (sibling, not nested).
+    // /<del[^>]*>[^<]*<ins/ would match a nested case but not a sibling case
+    // because [^<]* stops before the closing </del> tag.
+    expect(html).not.toMatch(/<del[^>]*>[^<]*<ins/);
+  });
 });
 
 import { renderPlain } from "../src/trackChangesModel";