diff --git a/lib/chrome/coverage.js b/lib/chrome/coverage.js
index 063f77139..343c2ba07 100644
--- a/lib/chrome/coverage.js
+++ b/lib/chrome/coverage.js
@@ -2,7 +2,8 @@ import { getLogger } from '@sitespeed.io/log';
 
 const log = getLogger('browsertime.chrome.coverage');
 
-// Sum the union length of half-open [start, end) ranges.
+// Sum the union length of half-open [start, end) ranges. Used for the
+// flat, non-nested ranges that CSS rule-usage tracking returns.
 function unionLength(ranges) {
   if (ranges.length === 0) return 0;
   const sorted = ranges
@@ -24,6 +25,45 @@ function unionLength(ranges) {
   return total;
 }
 
+// Count used bytes for a script under detailed (block-level) V8
+// coverage. The CDP returns nested ranges: the outermost range is the
+// function body and inner ranges are sub-blocks. An inner range with
+// count == 0 inside a containing range with count > 0 represents dead
+// code in an otherwise-executed function — those bytes must read as
+// unused. A naive union of every range whose count > 0 is wrong: the
+// outer range alone covers the entire function, so inner count == 0
+// ranges get masked and zero-count branches disappear, making typical
+// modern bundles look as if every byte was executed. Walk every range
+// across every function from outermost to innermost (start ascending,
+// length descending), painting a per-byte coverage flag; inner ranges
+// overwrite outer ones, so the final flag at each byte reflects the
+// innermost range's count.
+export function usedJsBytes(scriptCoverage, totalBytes) {
+  const ranges = [];
+  for (const fn of scriptCoverage.functions) {
+    for (const r of fn.ranges) {
+      if (r.endOffset > r.startOffset) ranges.push(r);
+    }
+  }
+  if (ranges.length === 0) return 0;
+  ranges.sort((a, b) =>
+    a.startOffset === b.startOffset
+      ? b.endOffset - a.endOffset
+      : a.startOffset - b.startOffset
+  );
+  const used = new Uint8Array(totalBytes);
+  for (const r of ranges) {
+    const start = Math.max(0, r.startOffset);
+    const end = Math.min(totalBytes, r.endOffset);
+    if (end > start) used.fill(r.count > 0 ? 1 : 0, start, end);
+  }
+  let count = 0;
+  for (let i = 0; i < totalBytes; i++) {
+    if (used[i] === 1) count++;
+  }
+  return count;
+}
+
 function summarize(files) {
   let totalBytes = 0;
   let usedBytes = 0;
@@ -143,13 +183,7 @@ export class Coverage {
       const totalBytes = source.length;
       if (totalBytes === 0) continue;
 
-      const usedRanges = [];
-      for (const fn of script.functions) {
-        for (const r of fn.ranges) {
-          if (r.count > 0) usedRanges.push(r);
-        }
-      }
-      const usedBytes = unionLength(usedRanges);
+      const usedBytes = usedJsBytes(script, totalBytes);
       const unusedBytes = totalBytes - usedBytes;
       files.push({
         url: script.url,
diff --git a/test/unittests/coverageTest.js b/test/unittests/coverageTest.js
new file mode 100644
index 000000000..cb582a8bb
--- /dev/null
+++ b/test/unittests/coverageTest.js
@@ -0,0 +1,142 @@
+import test from 'ava';
+import { usedJsBytes } from '../../lib/chrome/coverage.js';
+
+test('whole function used — single range, count > 0', t => {
+  t.is(
+    usedJsBytes(
+      {
+        functions: [{ ranges: [{ startOffset: 0, endOffset: 100, count: 5 }] }]
+      },
+      100
+    ),
+    100
+  );
+});
+
+test('whole function unused — single range, count = 0', t => {
+  t.is(
+    usedJsBytes(
+      {
+        functions: [{ ranges: [{ startOffset: 0, endOffset: 100, count: 0 }] }]
+      },
+      100
+    ),
+    0
+  );
+});
+
+test('function called with dead branch — inner count=0 punches a hole', t => {
+  // The bug case: the outer range alone would mark the whole function as
+  // used, masking the inner dead branch. Innermost-wins must keep the
+  // 30..50 hole.
+  t.is(
+    usedJsBytes(
+      {
+        functions: [
+          {
+            ranges: [
+              { startOffset: 0, endOffset: 100, count: 5 },
+              { startOffset: 30, endOffset: 50, count: 0 }
+            ]
+          }
+        ]
+      },
+      100
+    ),
+    80
+  );
+});
+
+test('two sibling functions — one executed, one not', t => {
+  t.is(
+    usedJsBytes(
+      {
+        functions: [
+          { ranges: [{ startOffset: 0, endOffset: 50, count: 3 }] },
+          { ranges: [{ startOffset: 50, endOffset: 100, count: 0 }] }
+        ]
+      },
+      100
+    ),
+    50
+  );
+});
+
+test('nested function never called — overrides containing function', t => {
+  t.is(
+    usedJsBytes(
+      {
+        functions: [
+          { ranges: [{ startOffset: 0, endOffset: 100, count: 5 }] },
+          { ranges: [{ startOffset: 30, endOffset: 50, count: 0 }] }
+        ]
+      },
+      100
+    ),
+    80
+  );
+});
+
+test('nested function rescued from a containing dead branch', t => {
+  // The containing function ran, but it has a dead branch covering
+  // 20..60. A nested function whose body is at 35..45 with count > 0
+  // must override the dead branch — its bytes are used.
+  t.is(
+    usedJsBytes(
+      {
+        functions: [
+          {
+            ranges: [
+              { startOffset: 0, endOffset: 100, count: 5 },
+              { startOffset: 20, endOffset: 60, count: 0 }
+            ]
+          },
+          { ranges: [{ startOffset: 35, endOffset: 45, count: 2 }] }
+        ]
+      },
+      100
+    ),
+    70
+  );
+});
+
+test('no functions returns zero', t => {
+  t.is(usedJsBytes({ functions: [] }, 100), 0);
+});
+
+test('empty ranges returns zero', t => {
+  t.is(usedJsBytes({ functions: [{ ranges: [] }] }, 100), 0);
+});
+
+test('zero-length ranges are ignored', t => {
+  t.is(
+    usedJsBytes(
+      {
+        functions: [
+          {
+            ranges: [
+              { startOffset: 0, endOffset: 100, count: 1 },
+              { startOffset: 50, endOffset: 50, count: 0 }
+            ]
+          }
+        ]
+      },
+      100
+    ),
+    100
+  );
+});
+
+test('ranges past totalBytes are clamped', t => {
+  // A defensive case: V8 returning endOffset > script length must not
+  // walk off the end of the typed array.
+  t.is(
+    usedJsBytes(
+      {
+        functions: [{ ranges: [{ startOffset: 0, endOffset: 200, count: 1 }] }]
+      },
+      100
+    ),
+    100
+  );
+});