diff --git a/selection/shadow-dom/tentative/Selection-getComposedRanges-range-update.html b/selection/shadow-dom/tentative/Selection-getComposedRanges-range-update.html
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+++ b/selection/shadow-dom/tentative/Selection-getComposedRanges-range-update.html
@@ -0,0 +1,190 @@
+<!DOCTYPE html>
+<html>
+<body>
+<meta name="assert" content="Selection's getComposedRanges should be updated when its associated live range changes">
+<link rel="help" href="https://w3c.github.io/selection-api/#dom-selection-getcomposedranges">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<div id="light">Start outside shadow DOM</div>
+<div id="outerHost">outerHost
+<template shadowrootmode="open">
+  <slot></slot>
+  <div id="innerHost">innerHost
+   <template shadowrootmode="open">
+     <slot></slot>
+   </template>
+  </div>
+</template>
+</div>
+
+<script>
+
+const selection = getSelection();
+const outerHost = document.getElementById('outerHost')
+const outerRoot = outerHost.shadowRoot;
+const innerHost = outerRoot.getElementById('innerHost');
+const innerRoot = innerHost.shadowRoot;
+
+test(() => {
+  // Step 1: Setting a composed live range that crosses boundaries
+  selection.setBaseAndExtent(light.firstChild, 10, innerHost.firstChild, 5);
+  const liveRange = selection.getRangeAt(0);
+  const composedRange = selection.getComposedRanges({ shadowRoots: [outerRoot, innerRoot] })[0];
+
+  assert_equals(liveRange.startContainer, innerHost.firstChild);
+  assert_equals(liveRange.startOffset, 5);
+  assert_equals(liveRange.endContainer, innerHost.firstChild);
+  assert_equals(liveRange.endOffset, 5);
+
+  assert_equals(selection.anchorNode, innerHost.firstChild);
+  assert_equals(selection.anchorOffset, 5);
+  assert_equals(selection.focusNode, innerHost.firstChild);
+  assert_equals(selection.focusOffset, 5);
+
+  assert_equals(composedRange.startContainer, light.firstChild);
+  assert_equals(composedRange.startOffset, 10);
+  assert_equals(composedRange.endContainer, innerHost.firstChild);
+  assert_equals(composedRange.endOffset, 5);
+
+  // Step 2: Update the live range only using setEnd
+  liveRange.setEnd(innerHost.firstChild, 6);
+  const composedRange2 = selection.getComposedRanges({ shadowRoots: [outerRoot, innerRoot] })[0];
+
+  assert_equals(liveRange.startContainer, innerHost.firstChild);
+  assert_equals(liveRange.startOffset, 5);
+  assert_equals(liveRange.endContainer, innerHost.firstChild);
+  assert_equals(liveRange.endOffset, 6);
+
+  assert_equals(selection.anchorNode, innerHost.firstChild);
+  assert_equals(selection.anchorOffset, 5);
+  assert_equals(selection.focusNode, innerHost.firstChild);
+  assert_equals(selection.focusOffset, 6);
+
+  assert_equals(composedRange2.startContainer, light.firstChild);
+  assert_equals(composedRange2.startOffset, 10);
+  assert_equals(composedRange2.endContainer, innerHost.firstChild);
+  assert_equals(composedRange2.endOffset, 6);
+
+  // Step 3: selectNode() calls both setStart/setEnd
+  liveRange.selectNode(innerHost);
+  const composedRange3 = selection.getComposedRanges({ shadowRoots: [outerRoot, innerRoot] })[0];
+
+  assert_equals(liveRange.startContainer, outerRoot);
+  assert_equals(liveRange.startOffset, 3);
+  assert_equals(liveRange.endContainer, outerRoot);
+  assert_equals(liveRange.endOffset, 4);
+
+  assert_equals(selection.anchorNode, outerRoot);
+  assert_equals(selection.anchorOffset, 3);
+  assert_equals(selection.focusNode, outerRoot);
+  assert_equals(selection.focusOffset, 4);
+
+  assert_equals(composedRange3.startContainer, outerRoot);
+  assert_equals(composedRange3.startOffset, 3);
+  assert_equals(composedRange3.endContainer, outerRoot);
+  assert_equals(composedRange3.endOffset, 4);
+
+  // Step 4: collapse(false) calls setEnd only
+  liveRange.collapse();
+  const composedRange4 = selection.getComposedRanges({ shadowRoots: [outerRoot, innerRoot] })[0];
+
+  assert_equals(liveRange.startContainer, outerRoot);
+  assert_equals(liveRange.startOffset, 4);
+  assert_equals(liveRange.endContainer, outerRoot);
+  assert_equals(liveRange.endOffset, 4);
+
+  assert_equals(selection.anchorNode, outerRoot);
+  assert_equals(selection.anchorOffset, 4);
+  assert_equals(selection.focusNode, outerRoot);
+  assert_equals(selection.focusOffset, 4);
+
+  assert_equals(composedRange4.startContainer, outerRoot);
+  assert_equals(composedRange4.startOffset, 4);
+  assert_equals(composedRange4.endContainer, outerRoot);
+  assert_equals(composedRange4.endOffset, 4);
+}, 'modify getRangeAt() range.');
+
+test(() => {
+  // Step 1: Creating a live range and only setting its end/anchor
+  selection.removeAllRanges();
+  const liveRange = document.createRange();
+  liveRange.setEnd(innerHost.firstChild, 5);
+  const composedRanges = selection.getComposedRanges({ shadowRoots: [outerRoot, innerRoot] });
+
+  assert_equals(liveRange.startContainer, innerHost.firstChild);
+  assert_equals(liveRange.startOffset, 5);
+  assert_equals(liveRange.endContainer, innerHost.firstChild);
+  assert_equals(liveRange.endOffset, 5);
+
+  assert_equals(selection.anchorNode, null);
+  assert_equals(selection.anchorOffset, 0);
+  assert_equals(selection.focusNode, null);
+  assert_equals(selection.focusOffset, 0);
+
+  assert_equals(composedRanges.length, 0);
+
+  // Step 2: Add range to selection so range API updates will change selection
+  selection.addRange(liveRange);
+  const composedRange = selection.getComposedRanges({ shadowRoots: [outerRoot, innerRoot] })[0];
+
+  assert_equals(liveRange.startContainer, innerHost.firstChild);
+  assert_equals(liveRange.startOffset, 5);
+  assert_equals(liveRange.endContainer, innerHost.firstChild);
+  assert_equals(liveRange.endOffset, 5);
+
+  assert_equals(selection.anchorNode, innerHost.firstChild);
+  assert_equals(selection.anchorOffset, 5);
+  assert_equals(selection.focusNode, innerHost.firstChild);
+  assert_equals(selection.focusOffset, 5);
+
+  assert_equals(composedRange.startContainer, innerHost.firstChild);
+  assert_equals(composedRange.startOffset, 5);
+  assert_equals(composedRange.endContainer, innerHost.firstChild);
+  assert_equals(composedRange.endOffset, 5);
+}, 'modify createRange() range added to selection after setEnd call.');
+
+test(() => {
+  // Step 1: Creating a live range and only setting its end/anchor
+  selection.removeAllRanges();
+  const liveRange = document.createRange();
+  // Add range to selection so range API updates will change selection
+  selection.addRange(liveRange);
+  liveRange.setEnd(innerHost.firstChild, 5);
+  const composedRange = selection.getComposedRanges({ shadowRoots: [outerRoot, innerRoot] })[0];
+
+  assert_equals(liveRange.startContainer, innerHost.firstChild);
+  assert_equals(liveRange.startOffset, 5);
+  assert_equals(liveRange.endContainer, innerHost.firstChild);
+  assert_equals(liveRange.endOffset, 5);
+
+  assert_equals(selection.anchorNode, innerHost.firstChild);
+  assert_equals(selection.anchorOffset, 5);
+  assert_equals(selection.focusNode, innerHost.firstChild);
+  assert_equals(selection.focusOffset, 5);
+
+  assert_equals(composedRange.startContainer, document);
+  assert_equals(composedRange.startOffset, 0);
+  assert_equals(composedRange.endContainer, innerHost.firstChild);
+  assert_equals(composedRange.endOffset, 5);
+
+  // Step 2: Update the live range by setting its start/focus
+  liveRange.setStart(light.firstChild, 10);
+  const composedRangeAfter = selection.getComposedRanges({ shadowRoots: [outerRoot, innerRoot] })[0];
+
+  assert_equals(liveRange.startContainer, light.firstChild);
+  assert_equals(liveRange.startOffset, 10);
+  assert_equals(liveRange.endContainer, light.firstChild);
+  assert_equals(liveRange.endOffset, 10);
+
+  assert_equals(selection.anchorNode, light.firstChild);
+  assert_equals(selection.anchorOffset, 10);
+  assert_equals(selection.focusNode, light.firstChild);
+  assert_equals(selection.focusOffset, 10);
+
+  assert_equals(composedRangeAfter.startContainer, light.firstChild);
+  assert_equals(composedRangeAfter.startOffset, 10);
+  assert_equals(composedRangeAfter.endContainer, innerHost.firstChild);
+  assert_equals(composedRangeAfter.endOffset, 5);
+}, 'modify createRange() range added to selection before setStart/setEnd calls.');
+</script>
