Every single day. For example, here's this great IM conversation I just had with Scott. Maybe it's old news to you, but I was severly out of the loop:
[14:09] FHBen: Yo, CSS-guru-- what's the difference between <br> and <p>, fo' reals, yo?
[14:09] FHBen: I'm all usin 'em interchangably an' shit...
[14:09] EP123NTER: ack!
[14:09] FHBen: ?
[14:09] EP123NTER: several things...
[14:10] EP123NTER: first, <br> is now <br /> - its a self-closing tag. and it is the visual equivalent of \n
[14:10] FHBen: ooh, ok.
[14:10] EP123NTER: <p> must be closed with </p> in order to be valid xml.
[14:11] EP123NTER: and it generally creates the visual equivalent of \n\n.
[14:11] EP123NTER: although you can modify the margins of <p> in you css to get whatever spacing you want.
[14:11] FHBen: Holy hell, I had no idea.
[14:12] EP123NTER: well, im glad that we cleared that up :)