Here ya go, about as minimal as I can imagine:

\setupbackend [export=yes]
\setupexport  []
\starttext
!
\stoptext

and in the resulting export styles directory, I get:

g:\yoiks-export\styles>grep "namespace\|weight.*italic\|text-width" *.css
yoiks-defaults.css:@namespace context url('http://www.pragma-ade.com/context/export') ;
yoiks-defaults.css:     font-weight : italic ;
yoiks-images.css:@namespace context url('%namespace%') ;
yoiks-styles.css:@namespace context url('%namespace%') ;
yoiks-styles.css:    text-width : justify !important ;
yoiks-templates.css:@namespace context url('%namespace%') ;

So:

• three unexpanded %namespace%s,
• one invalid property (text-width),
• and one incorrect property (font-weight should be font-style).
  

The namespace and text-width issues are now fixed, but the font-weight : italic fault in default.css remains.

11     document, div.document     inherited is not a hyphens value : inherited

That validator (and others) also report a number of issues for the @counter-style rule, which seems to have changed significantly since this CSS was developed. Validators report a number of smaller issues as well.

I understand that export output is a low-priority feature.

--
Rik