Pretty clean. I can't help but think that Yahoo! is just trying to roll their own Twitter Bootstrap.
I love Bootstrap but it's a pain to build on top of it, because it's so opinionated in its design.
yahoo.com: 58 requests, 336k. wsj.com: 123 requests, 1410k
I'm a fan of making things smaller and use GWT to compile and compress versions of sites for each browser and format to achieve the bare minimum, but starting out at the CSS lib might be premature optimization give that it's not where most of the fat is. Plus, next to no site is going to use just that lib and their CSS will surely grow well in excess of that 5k.