base64 encoding a canvas does not actually specify base64 encoding

The algorithm for encoding a canvas element to abse64 effectively says to convert it into a data url and then remove the url part, leaving only the data part after the comma.

This is a weird way of specifying things (print does it much better, it simply says to base64 encode the pdf file - why not just say base64 encode the png file?).

The real problem is, however, that this does not result in base464 encoded data - the algorithm only says to encode it in "a" data url, but fails to enforce that it must be a base64 data url (as opposed to uri-escaped or any other encodings which are legal for data urls).

Start with the canvas element encoding algorithm and compare its data URL wording with the print algorithm mentioned in the issue. Update the specification so canvas output is explicitly base64 encoded rather than allowing any valid data URL encoding. Confirm the revised text clearly requires base64 data after the URL prefix is removed.