Because there is no controlslist
attribute on the audio
element in the current official HTML5 standard.
FWIW I can't find any mention of a controlslist
attribute on the audio
element. The linked question refers to a controlsList
*1 attribute on the video
element (although I would expect all media elements to share the same attributes).
(*1 The linked question uses a capital "L" in "List", throughout the page, perhaps reflecting the JS property name, although the HTML attribute would be case-insensitive.)
The Mozilla.org docs for the audio
element have no mention of a controlslist
attribute. However, for the video
element they do list a controlslist
attribute, but it is listed as:
This is an experimental API that should not be used in production code.
So, it would seem this is an "experimental" feature which has (as yet) spotty browser support (notably missing from Firefox 84-86, Safari 14, iOS Safari and Firefox Android 83 and... IE11). However, browser's that don't support this may not offer a download button anyway (I don't know - I've not looked into this). As always, test test test in the browsers you intend to support.
Consequently, if you want to use this attribute then you'll have to sacrifice W3C validation.
<!DOCTYPE html><html lang="ja" class="js" dir="ltr" prefix="og: https://ogp.me/ns#">