Audio fallback hidden

[iseulde]

Introduced in [28182].

Add visibility: hidden as an inline style to <audio> tags, there is a race with the stylesheet which can get enqueued in the body and loaded in the footer.

However it also makes the fallback invisible in case it is needed.

[iseulde]

If the script is enqueued at the bottom, the native player will be visible shortly before being replaced by the MediaElement player. Right the space is empty before being replaced.

[iseulde]

In 37556:

Media: unhide audio fallback

Introduced in [28182].
Fixes #36888.

[iseulde]

In 37557:

Media: adjust test after [37556]

See #36888.