It seems the atob() flavor of base64 is also used by data URLs, so Fetch will need it. Should the algorithm move to Fetch, stay in HTML, or move elsewhere (IDL and Infra come to mind)?

We'll also need to make the algorithm return a byte sequence. (If we switch to ByteString as suggested in #2911 that change won't be observable for atob(). It's only observable for btoa() due to the exception change.)