Validación HTML: CSS: Unknown pseudo-element or pseudo-class “::file-selector-button”.

Acerca de este problema de CSS

The ::file-selector-button pseudo-element targets the button inside an <input type="file"> element — the part users click to open the file picker dialog. It is supported in all major modern browsers and is part of the CSS Pseudo-Elements Level 4 specification.

The W3C CSS Validator has not yet been updated to recognize ::file-selector-button, which causes it to flag the pseudo-element as invalid. This is a known bug that has been reported on GitHub. Because the validator’s CSS grammar definitions lag behind the living standards, certain newer — but fully standardized — features trigger false positives.

Before ::file-selector-button was standardized, browsers used vendor-prefixed versions like ::-webkit-file-upload-button and ::-ms-browse. The unprefixed ::file-selector-button replaced these and is now the recommended approach.

What you should do

Since this is a validator limitation rather than an actual code issue, you can safely ignore this warning. Your CSS is standards-compliant. If you want to suppress the warning in a CI/CD pipeline or validation report, you can add a comment noting the false positive, but do not remove or change the pseudo-element — it is correct.

Browser support

::file-selector-button is supported in Chrome 89+, Firefox 82+, Safari 14.1+, and Edge 89+. For older browsers, you may include the vendor-prefixed versions as fallbacks, though these will also trigger validator warnings.

Examples

CSS that triggers the validator warning

::file-selector-button {
  background-color: #4a90d9;
  color: white;
  border: none;
  padding: 8px 16px;
  border-radius: 4px;
  cursor: pointer;
}

This CSS is valid and correct despite the validator warning. It styles the file selector button with a custom background color, text color, and border radius.

Scoping to a specific input

input[type="file"]::file-selector-button {
  background-color: #4a90d9;
  color: white;
  border: none;
  padding: 8px 16px;
  border-radius: 4px;
}

input[type="file"]::file-selector-button:hover {
  background-color: #357abd;
}

Including vendor-prefixed fallbacks for older browsers

input[type="file"]::-webkit-file-upload-button {
  background-color: #4a90d9;
  color: white;
  border: none;
  padding: 8px 16px;
}

input[type="file"]::file-selector-button {
  background-color: #4a90d9;
  color: white;
  border: none;
  padding: 8px 16px;
}

The vendor-prefixed version is placed first so that the standard ::file-selector-button rule takes precedence in browsers that support both. Note that the vendor-prefixed versions will also trigger validator warnings for the same reason.

Corresponding HTML

<label for="upload">Choose a file:</label>
<input type="file" id="upload" name="upload">

There is nothing to fix in your HTML or CSS. This is purely a validator false positive that will be resolved when the W3C Validator updates its CSS grammar definitions.