Speed of CSS

Answer 1

According to the Mozilla Developer Center, the most efficient selector for this case would be simply .third

In fact, they state there, explicitly, "Don't qualify Class Rules with tag names". In general, for greatest efficiency, use the simplest selector that matches. That's why .third beats span.third beats .second span.third, etc.

I can't tell what Gumbo's conclusion is meant to be, but Ólafur appears to be drawing the conclusion from it that using more stuff in the selector makes the CSS parsing more efficient, when, in fact, it's just the opposite (assuming other CSS parsing engines work similarly to Mozilla's.)