Using SCSS and frameworks makes it distressingly easy to hit these limits.
Internet Explorer has terrible support for pseudo selectors. With Selectivizr, we can make that support a little nicer and do some fun stuff with CSS!
I recently found out that IE can't handle more than 4,095 selectors in a single CSS file. Here's my work-around.
When we first started building up our web test suite using Watin, we quickly started getting this error: System.Runtime.InteropServices.COMException
Hurdles aside, using WatiN and SpecFlow for system testing has worked out well for a testing problem that has historically been hard-won, if won at all.