Wow, for a "three things you need to know" title, this article was surprisingly informative. Combining attr with :before is a nice trick; I'll be on the lookout for my own reasons to use that. And calc sounds great.
Really cool stuff.
* Styling important content links for print. An anchor tag isn't clickable in print media, potentially leaving important citation information out. You can use attr to put it back in.
* Sometimes you find yourself debugging device/browser where you don't have a powerful inspector/debugger. Being able to make attribute data visible can help.