Upper case selectors

A user, who visited a web that I designed, had sent an email to
me, he says that my CSS code have an accessibility problem.
My code is XTML 1.0 valid, and CSS valid. In the CSS, I have
selectors like “DIV.uno”. He sent the follow paragraph of XHTML
specification:

“C.13. Cascading Style Sheets (CSS) and XHTML
The Cascading Style Sheets level 2 Recommendation [CSS2] defines
style
properties which are applied to the parse tree of the HTML or
XML documents.
Differences in parsing will produce different visual or aural
results,
depending on the selectors used. The following hints will reduce
this effect
for documents which are served without modification as both
media types:

1. CSS style sheets for XHTML should use lower case element and
attribute
names.
2. ..."

The link to style sheet on my page haven’t media declaration:
<link rel="stylesheet" href="xxx.css" title="estilobase"
type="text/css" />

Is it an accessibility problem for anyone, now?
What about the future?
If it’s an accessibility problem, which priority is it?


Thanks in advance,
Ricardo Sánchez