WTF, HTML and CSS?
I'm surprised this didn't mention stacking contexts, which threw me for a loop a couple times:<p><a href="http://philipwalton.com/articles/what-no-one-told-you-about-z-index/" rel="nofollow">http://philipwalton.com/articles/what-no-one-told-you-about-...</a><p><a href="https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Under...</a>