<p>The HTML standard defines a clear separation of concerns between CSS (presentation) and HTML (semantics or structure).</p>

<p>Does anyone use a coding standards document for CSS and XHTML that has clauses which help to maintain this separation? What would be good clauses to include in such a coding standards document?</p>

We don't have a physical document we all adhere to where I work. There are a number of guidelines we try and keep in mind but there isn't really enough information to require a physcial document.
This article
sums them up these guidelines pretty well.  You may also consider formatting your
```
CSS
```
to make it easier to read.
Smashing Magazine
has a great article on this subject.