For me too, without question, valid html. Through W3C Unicorn I get 3 back links per valid page from a PR10 domain (root). This usually propels my client sites to first page results within weeks of launch with almost 0 back links.
And you have to link to them in a way that your page becomes a bookmark in their database so it is always available even without being referred by your domain.
I don't mind letting this be known because to take advantage of it one has to produce valid code or there will be no back links on the W3C page. Google is somehow capable of finding those bookmarked back links within W3C and gives (superb) credit for them.