@mdo posted a nice list of HTML and CSS WTFs, common traps and pitfalls today - WTF, HTML and CSS?
My all-time favourite is still the box model of CSS, which fortunately can now be remedied by the glorious
box-sizing property. Oooh, the headaches this little bugger caused...
On a related note: to check your fluency in CSS selectors, go have a look at CSS Diner.