Created attachment 309045 [details]
reduced test case
To reproduce just open the attached html test case.
In Safari Technology Preview Release 29 (Safari 10.2, WebKit 12604.1.19.0.1) the iframe reloads forever, the audio in it is never playable.
With Chrome 58.0.3029.81 beta (64-bit) and Firefox 53.0 the iframe only loads once and the audio is playable.
Assuming this was a general bug with dynamically generated documents in iframes, I tried replacing the media element with an img, but the issue does not reproduce.
There's likely something specific about media causing onload to fire twice, so the iframe keeps getting reloaded over and over.
Note: This isn't a recent regression in STP as it also reproduces in Safari 10.1
(In reply to Brady Eidson from comment #4)
> Note: This isn't a recent regression in STP as it also reproduces in Safari
Also reproduced in Safari 10.0
Still reproduces with STP Release 62 (Safari 12.1, WebKit 13607.1.1)