What is the issue with the HTML Standard?

Currently the file upload control rendering just says:

followed by a button that, when activated, provides the user with a file picker from which the selection can be changed. The button is expected to use button layout and match the '::file-selector-button' pseudo-element.

WebKit and Chromium implement this using <input type="button"> and Firefox uses <button>. Historically this wasn't observably different. But CSS now defines ::file-selector-button as an element backed pseudo element. Input buttons and real buttons don't behave the same for various pseudo elements. So the spec being ambiguous here could lead to interop issues.

• ::before and ::after don't work on input buttons in any browser, but do on real buttons.
• ::first-letter doesn't work on input buttons in any browser, but does on real buttons.
• ::first-line doesn't work on input buttons in Firefox, but does on real buttons.