Selector Performance

Much has been written and discussed over the years with respect to selector performance -- however, the best practises have also evolved over time as performance inside of jQuery has improved, and browsers have improved (especially the advent of QSA). A lot of the literature out there on this comes from different points in history, which leads newcomers to get a skewed view of priorities. We should explain some of this history and use this article as the clearing house on what is currently "best," things like

• not being overly specific in selectors
• avoiding jQuery-only psuedoselectors
• including quotes in attribute selectors to get into QSA
• Sizzle vs. QSA
• thoughts on balancing performance in fast modern browsers in QSA with oldIE
• where selector performance fits into the general app performance picture

(Hoping to get a lift here from @timmywil after the Sizzle rewrite for 1.8. @crowchick proposed some content for this in #54 that could certainly be fleshed out here to anyone's heart's content)

Start by reviewing the selector-performance topics listed in the issue, including QSA, Sizzle, jQuery-only pseudo-selectors, quoted attribute selectors, old IE, and broader application performance. Done means producing a current, historically grounded article that consolidates these recommendations for newcomers.