This is more of a featuer request I suppose but ehre goes:
If you are embedding serendipity into another web page, then the the stylesheet for it becomes vary problematic. For instance the defaul style sheet set some styles on the A and TD tags. this means that the web page that serendipity is embbeded in gets their A tags and TD tags modified.
Instead of using global css modifiers I suggest that you use A.serendipty and TD.serendipity instead of golbaly changing TD and A (this also would go for other tags like H1-4, input and select). The only think that you would really have to change is setting the class attribute of the tags that serendipity outputs and then update the stylesheets.
This would make serendipty work alot better in emmbeded mode.