It seems that the macros in t.tex are being written out as XML elements, verbatim. It is my understanding that these XML elements, however, do not conform to the minimal content models associated with XHTML 1.1.
What needs to happen to take a minimal ConTeXt file (such as the attached) to produce a minimum viable EPUB that:
Generates XHTML headers (including <!DOCTYPE and <html...>)
Produces images as img tags, rather than float tags.
Uses typical XHTML tags for <body> elements (e.g., <ol> for ordered lists).
Ideally, I would like to do something such as:
context t.tex
mtxrun --script epub --make t.specification
to generate an EPUB that passes validation of epubcheck, with an output XHTML file that more closely matches the XHTML specification.