Oh man, I could write volumes on this.

One of my top pet peeves is people using "XHTML" (sometimes even with an xml declaration) and still using:
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

Why, oh why, oh WHY? Why are you setting your content type to text/html if (you claim) you're using xhtml?

Fake XHTML bugs me so much, and its so widespread. Why do people use it? Why? I can't ask that enough.

As far as <br /> being used in forum posts, I could actually see an argument being made that this is potentially proper use of the <br /> element. Sure, it's not specified, but when a person writes a single line in a forum post and then adds a few new lines, do they really think of it as a separate grouping of text? a separate element? Probably not. I'm guessing most people certainly don't think of it as a separate "paragraph" (although I know the specification's definition of "paragraph" differs from the more common and colloquial definition of "paragraph").

One thing that bugs me about CSS is that the general consensus says that absolute positioning is bad to use. Why is it bad? Because it has a few bugs in IE7? Float has bugs in IE7 as well (float: right; is profoundly screwed up). Absolute positioning is specified, as long as you're using it properly, you have nothing to worry about. Float wasn't even meant to be used for positioning layouts (!) and people say that absolute positioning is the bad guy?

While I'm talking about CSS, another thing that bugs me is people who put a newline after *every* property in their css. Why? It makes it so unreadable and so unorganized when you end up having thousands of lines when it could be reduced down to less than 500 lines. I get the worst feeling when I open up a css file to take a look at it, and my scrollbar shrinks down to a single pixel in height.