the Epiphany Set gives 5% per Rank to increase your Caps. Attack Speed has a cap of 60. if I have Epiphany (5), that will increase my cap by 25%. 60 x 25% (or 60 x 1.25) = 75%. if you had Epiphany (8) the cap for Attack Speed would be 84%. but that would leave you with only 2 more spaces for other Sets, 3 if you have a Pet with a Set on it. most people just use the Epiphany (5).
whether you are using Epiphany or not, if you go over Cap, anything over isn't counted. so if you had a Crystal +225% Critical Damage and 2x Epic 75% Critical Damage, you would have +375% Critical Damage. but the Cap is 350, so 25% is wasted. but in this case, it is ok, as this is one of the affixes that it is hard to make a perfect cap with. most affixes can get you to the cap perfectly.