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diff --git a/testing/web-platform/tests/html/semantics/selectors/pseudo-classes/default.html b/testing/web-platform/tests/html/semantics/selectors/pseudo-classes/default.html
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+++ b/testing/web-platform/tests/html/semantics/selectors/pseudo-classes/default.html
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+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Selector: pseudo-classes (:default)</title>
+<link rel="author" title="Denis Ah-Kang" href="mailto:denis@w3.org" id=link1>
+<link rel=help href="https://html.spec.whatwg.org/multipage/#pseudo-classes" id=link2>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="utils.js"></script>
+<div id="log"></div>
+<form>
+  <button id=button1 type=button>button1</button>
+  <button id=button2 type=submit>button2</button>
+</form>
+<form>
+  <button id=button3 type=reset>button3</button>
+  <button id=button4>button4</button>
+</form>
+<button id=button5 type=submit>button5</button>
+<form id=form1>
+  <input type=text id=input1>
+</form>
+<input type=text id=input2 form=form1>
+<form>
+  <input type=submit id=input3>
+  <input type=submit id=input4>
+</form>
+<form>
+  <input type=image id=input5>
+  <input type=image id=input6>
+</form>
+<form>
+  <input type=submit id=input7>
+</form>
+<input type=checkbox id=checkbox1 checked>
+<input type=checkbox id=checkbox2>
+<input type=checkbox id=checkbox3 default>
+<input type=radio name=radios id=radio1 checked>
+<input type=radio name=radios id=radio2>
+<input type=radio name=radios id=radio3 default>
+<select id=select1>
+ <optgroup label="options" id=optgroup1>
+  <option value="option1" id=option1>option1
+  <option value="option2" id=option2 selected>option2
+</select>
+<dialog id="dialog">
+  <input type=submit id=input8>
+</dialog>
+<form>
+  <button id=button6 type='invalid'>button6</button>
+  <button id=button7>button7</button>
+</form>
+<form>
+  <button id=button8>button8</button>
+  <button id=button9>button9</button>
+</form>
+
+
+<script>
+  testSelectorIdsMatch(":default", ["button2", "button4", "input3", "input5", "input7", "checkbox1", "radio1", "option2", "button6", "button8"], "':default' matches <button>s that are their form's default button, <input>s of type submit/image that are their form's default button, checked <input>s and selected <option>s");
+
+  document.getElementById("button1").type = "submit"; // change the form's default button
+  testSelectorIdsMatch(":default", ["button1", "button4", "input3", "input5", "input7", "checkbox1", "radio1", "option2", "button6", "button8"], "':default' matches dynamically changed form's default buttons");
+
+</script>