I saw a question about some html come acrossed one of the maililng lists I'm on this morning, and thought I'd share some about it! Since it was a relatively new piece of knowledge to me, I thought maybe it would be helpful to someone.
The <base> tag: The base element defines a default reference to external resources.
There are definite advantages to being able to specify a default location. It means that you don't need to use absolute URLs to images when using PHP or SSI headers which will be used acrossed multiple directories and subdirectories. Same goes for templates on journal/weblogs, even when you are using the design throughout the system even in archive directories. I just stick this tag inside my head before my stylesheet link:
<base href="http://your-site.com/images">
Then the src to a image only looks like this:
<img src="smile.gif">
So basically, it just makes sure that your pages know where they are in the directory structure instead of using a lot of relative paths that look like ../../image and trying to keep track of where in the structure you are! Isn't it nice to know something that could make your html files smaller or save a few keystrokes?