Simon Willison’s Weblog

Like DOCTYPE switching did in 2000, version targeting negates the vendor argument that existing behaviors can't be changed for fear of breaking web sites. If IE8 botches its implementation of some CSS property or DOM method, the mistake can be fixed in IE9 without breaking sites developed in the IE8 era. This actually makes browser vendors more susceptible to pressure to fix their bugs, and less fearful of doing so.

— Eric Meyer