file-selector-button-after-part.html

<!doctype html>
<title>::file-selector-button allows to customize the button in &lt;input type=file&gt;</title>
<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/5049">
<link rel="help" href="https://drafts.csswg.org/css-pseudo/#file-selector-button-pseudo">
<link rel="mismatch" href="file-selector-button-001-notref.html">
<style>
#host::part(file-input)::file-selector-button {
  border: 1px solid green;
}
</style>
<div id="host"></div>
<script>
  document.getElementById("host").attachShadow({ mode: "open" }).innerHTML = `
    <input type=file part=file-input>
  `;
</script>

chromium/third_party/blink/web_tests/external/wpt/css/css-pseudo/file-selector-button-after-part.html