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diff --git a/testing/web-platform/tests/css/css-highlight-api/HighlightRegistry-iteration-with-modifications.html b/testing/web-platform/tests/css/css-highlight-api/HighlightRegistry-iteration-with-modifications.html
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+++ b/testing/web-platform/tests/css/css-highlight-api/HighlightRegistry-iteration-with-modifications.html
@@ -0,0 +1,109 @@
+<!doctype html>
+<title>HighlightRegistry iteration with insertions and deletions inbetween</title>
+<link rel="help" href="https://drafts.csswg.org/css-highlight-api-1/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+  'use strict';
+  let customHighlight1 = new Highlight();
+  let customHighlight2 = new Highlight();
+  let highlightName1 = "example1";
+  let highlightName2 = "example2";
+
+  // Test insertions using .add
+  test(() => {
+    let iterator = CSS.highlights[Symbol.iterator]();
+    CSS.highlights.set(highlightName1, customHighlight1);
+    let element = iterator.next();
+    assert_true(element.done, 'The iteration ends although we added a new Highlight after starting the iteration');
+    assert_equals(element.value, undefined, 'A Highlight added after starting the iteration is not found during the current iteration');
+  }, 'HighlightRegistry iteration is not modified when a new Highlight is added after starting the iteration');
+
+  CSS.highlights.clear();
+
+  test(() => {
+    CSS.highlights.set(highlightName1, customHighlight1);
+    let iterator = CSS.highlights[Symbol.iterator]();
+    CSS.highlights.set(highlightName2, customHighlight2);
+    let element = iterator.next();
+    assert_false(element.done, 'The iteration doesn\'t end although there was a second Highlight added to the HighlightRegistry after starting the iteration');
+    assert_equals(element.value[0], highlightName1, 'The highlight name that was pointed to by the iterator is returned although a second Highlight was added after starting the iteration');
+    assert_equals(element.value[1], customHighlight1, 'The Highlight that was pointed to by the iterator is returned although a second Highlight was added after starting the iteration');
+    element = iterator.next();
+    assert_true(element.done, 'The iteration ends after going through all the Highlights that were in the HighlightRegistry when the iteration started although there was a Highlight addition after starting the iteration');
+    assert_equals(element.value, undefined, 'A Highlight added after starting the iteration is not found during the current iteration');
+  }, 'HighlightRegistry iteration is not modified when a new Highlight is added after starting the iteration with one Highlight in the HighlightRegistry');
+
+  CSS.highlights.clear();
+
+  // Test deletions using .delete
+  test(() => {
+    CSS.highlights.set(highlightName1, customHighlight1);
+    let iterator = CSS.highlights[Symbol.iterator]();
+    CSS.highlights.delete(highlightName1);
+    let element = iterator.next();
+    assert_false(element.done, 'The iteration doesn\'t end although the Highlight that was pointed to by the iterator was deleted');
+    assert_equals(element.value[0], highlightName1, 'The highlight name that was pointed to by the iterator is returned although it was deleted after starting the iteration');
+    assert_equals(element.value[1], customHighlight1, 'The Highlight that was pointed to by the iterator is returned although it was deleted after starting the iteration');
+    element = iterator.next();
+    assert_true(element.done, 'The iteration ends after going through all the highlights although the Highlight that was pointed to by the iterator was deleted after starting the iteration');
+    assert_equals(element.value, undefined, '.next() returns undefined when the iteration ends');
+  }, 'HighlightRegistry iteration is not modified when the Highlight that was pointed to by the iterator was deleted after starting the iteration');
+
+  CSS.highlights.clear();
+
+  test(() => {
+    CSS.highlights.set(highlightName1, customHighlight1);
+    CSS.highlights.set(highlightName2, customHighlight2);
+    let iterator = CSS.highlights[Symbol.iterator]();
+    CSS.highlights.delete(highlightName2);
+    let element = iterator.next();
+    assert_false(element.done, 'The iteration doesn\'t end although the Highlight following to the one that was pointed to by the iterator was deleted');
+    assert_equals(element.value[0], highlightName1, 'The highlight name that was pointed to by the iterator is returned as it should although the next Highlight was deleted immediately after starting the iteration');
+    assert_equals(element.value[1], customHighlight1, 'The Highlight that was pointed to by the iterator is returned as it should although the next Highlight was deleted immediately after starting the iteration');
+    element = iterator.next();
+    assert_false(element.done, 'The iteration doesn\'t end when you call .next twice since the beginning of the iteration although the second Highlight was deleted');
+    assert_equals(element.value[0], highlightName2, 'The highlight name that was pointed to by the iterator is returned as it should although the next Highlight was deleted immediately after starting the iteration');
+    assert_equals(element.value[1], customHighlight2, 'The Highlight that was pointed to by the iterator is returned as it should although the next Highlight was deleted immediately after starting the iteration');
+    element = iterator.next();
+    assert_true(element.done, 'The iteration ends after going through all the highlights although the second Highlight was deleted immediately after starting the iteration');
+    assert_equals(element.value, undefined, '.next() returns undefined when the iteration ends');
+  }, 'HighlightRegistry iteration is not modified when the Highlight that was immediately after the one pointed to by the iterator was deleted after starting the iteration');
+
+  CSS.highlights.clear();
+
+  test(() => {
+    CSS.highlights.set(highlightName1, customHighlight1);
+    CSS.highlights.set(highlightName2, customHighlight2);
+    let iterator = CSS.highlights[Symbol.iterator]();
+    let element = iterator.next();
+    assert_false(element.done, 'The iteration doesn\'t end when there are still two Highlights to visit');
+    assert_equals(element.value[0], highlightName1, 'The highlight name that was pointed to by the iterator is returned as it should');
+    assert_equals(element.value[1], customHighlight1, 'The Highlight that was pointed to by the iterator is returned as it should');
+    CSS.highlights.delete(highlightName1);
+    element = iterator.next();
+    assert_false(element.done, 'The iteration doesn\'t end when the Highlight previously visited is deleted and there is still a Highlight to visit');
+    assert_equals(element.value[0], highlightName2, 'The highlight name that was pointed to by the iterator is returned as it should although the previous Highlight was deleted after calling .next');
+    assert_equals(element.value[1], customHighlight2, 'The Highlight that was pointed to by the iterator is returned as it should although the previous Highlight was deleted after calling .next');
+    element = iterator.next();
+    assert_true(element.done, 'The iteration ends after going through all the highlights although the first Highlight was deleted after the first call to .next');
+    assert_equals(element.value, undefined, '.next() returns undefined when the iteration ends');
+  }, 'HighlightRegistry iteration is not modified when a Highlight that was already visited is deleted and there are still Highlights to visit');
+
+  CSS.highlights.clear();
+
+  // Test deletions using .clear
+  test(() => {
+    CSS.highlights.set(highlightName1, customHighlight1);
+    let iterator = CSS.highlights[Symbol.iterator]();
+    CSS.highlights.clear();
+    let element = iterator.next();
+    assert_false(element.done, 'The iteration doesn\'t end although the Highlight that was pointed to by the iterator was deleted using .clear()');
+    assert_equals(element.value[0], highlightName1, 'The highlight name that was pointed to by the iterator is returned although it was deleted using .clear() after starting the iteration');
+    assert_equals(element.value[1], customHighlight1, 'The Highlight that was pointed to by the iterator is returned although it was deleted using .clear() after starting the iteration');
+    element = iterator.next();
+    assert_true(element.done, 'The iteration ends after going through all the highlights although the Highlight that was pointed to by the iterator was deleted using .clear() after starting the iteration');
+    assert_equals(element.value, undefined, '.next() returns undefined when the iteration ends');
+  }, 'HighlightRegistry iteration is not modified when the Highlight that was pointed to by the iterator was deleted using .clear() after starting the iteration');
+
+</script>