Custom elements in XHTML

I've been searching the internet for days now and can't find a single
answer to my question.
 
When using XHTML and CSS, I find myself creating a lot of classes in DIV
and SPAN elements, such as;
<span class="xhtmlCode">.</span>
 
Now with XHTML being _extensible_ I would have thought I could make my
own element such as;
<xhtmlcode>.</xhtmlcode>
 
Then style it with CSS. This would be much easier to read, but in my
research, it doesn't seem possible.
 
I would be grateful of any suggestions and an example if this is
possible.
 
Yours truly,
Barry Broom