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