I was recently building a "mega menu" (a really big drop-down menu panel) for primary navigation, so I decided to re-read the Jakob Nielsen Alertbox regarding them. Apparently the mega menu is the only form of drop-down menu to perform well under user testing.
In the discussion of timing for the interaction, I was reminded of how only a few years ago it seemed that creating drop-down menus using only the CSS :hover pseudoclass was the trend, and how that pretty much violates all of the recommendations:
Nielsen goes on to say, regarding mouseover:
[T]he mouse should remain stationary for 0.5 seconds before you display anything that's hover-dependent, such as a mega drop-down or a tooltip. Violating this guideline will make the screen flicker insufferably when users move the mouse.
Don't bother making the drop-down itself accessible. Instead, make each top-level menu choice clickable, leading to a regular Web page where you present all drop-down options in plain, fully accessible HTML.
It's almost too obvious. CSS drop-downs, you sure were a fun technical exercise, but I'm happy to bid you farewell in favor of a better experience for users (not to mention less cross-browser-induced head trauma!).