I created an ebook using this online
url-to-epub tool, then edited it to not look horrible using Calibre. When I try to upload it to My Books on Google Play, it says "processing failed". I used this
epub validator tool and it finds a bunch of errors (e.g.
Error while parsing file: attribute "class" not allowed here; expected attribute "version" or "xml:lang"), yet the file reads just fine in Calibre.
Is there some tool that leaves the contents untouched but fixes the file so that it's properly formatted and can be read by programs that are more picky than Calibre?