A couple of small issues with the css for epub output.

First, back-exp.lua creates, in \jobname-styles.css

@namespace context url('%namespace%') ;

This should probably be:

@namespace context url('http://www.pragma-ade.com/context/export') ;

That file also has:

document,
%namespace%div.document {
    font-size  : %size% !important ;
    max-width  : %width% !important ;
    text-width : %align% !important ;
    hyphens    : %hyphens% !important ;
}

which contains the invalid css property text-width. Perhaps that should be text-align or text-justify.

Something else appears to be creating, in \jobname-defaults.css:

pubfld[detail="title"],
div.pubfld.title {
    display     : inline ;
    font-weight : italic ;
}

which incorrectly assigns italic to font-weight when of course it  is a font-style as far as css is concerned. This can be seen in file export-example.css in the distribution.

--
Rik