chromium/third_party/blink/web_tests/fast/forms/stuff-on-my-optgroup.html

<select><optgroup label="Stuff"><option>One</option><script>document.write("<option>Two</option>")</script></optgroup></select><br>
<select><optgroup label="More stuff">Random text that should just be ignored but still be in the DOM.<option>One<option>Two</select>