The issue isn't what format the content is in, but that it's retrieved through client-side logic rather than as part of the query response.
Also, what's wrong with HTML/CSS that is in any way resolved by XML/XSLT? I'm genuinely curious why somebody would put their content into a format with no default, well-understood display semantics.
Maybe I misunderstood how the xml/xslt-paradigm works, but my idea was that you have a serverside api serving up raw data, and the display is controlled by some code on the clientside that can be easily customised. So the server could serve up something like
<shoe>
<images>
<image>foo.com/x.jpg</image>
<image>foo.com/y.jpg</image>
</images>
<description>This shoe is very advanced bla bla bla</description>
<fancyintroanimation>foo.com/intro1.swf</fancyintroanimation>
</shoe>
<shoe>
....
and the client could use the default xslt if they were happy with that, and a custom otherwise.
Also, what's wrong with HTML/CSS that is in any way resolved by XML/XSLT? I'm genuinely curious why somebody would put their content into a format with no default, well-understood display semantics.