Validity of Wordpress-Blog

Hi there,

I am a beginner with validation, so my question should be very easy. But I
did not find an answer yet.
I have my little blog http://www.rhine-mosel.com/ which runs with wordpress
2.8.4.
The validator on w3.org tells me that there is one error:

Attribute "role" is not a valid attribute. Did you mean "frameborder" or
"scrolling"?

<div id="header" role="banner">

Since wordpress is maybe the most common CMS on earth, is it a valid CMS,
generally spoken? My site is very basic and
I have not changed the code or s.th.

And the second question: If there is only one error, should I change the
code, or is this no big problem?

Thank you
Guido
http://www.rhine-mosel.com/