Its ridiculous to say Williams would probably beat Mayweather for many reasons
1. Who has Williams really foughten, Mragarito? Bah he's just an overrated brawler, also his style is nothing like Mayweather's
2. Who has Margarito foughten like Mayweather?
3. Mayweather has foughten guys who throw a ton of punches and guys who are tall, and he does well against both.
4. Mayweather is the best in thedivision and in the sport so its ridiculous that he would Probably lose to anyone near his division.

What Williams does have going for him
1. He might overwhelm Mayweather with his punch output if Mayweather doesn't use enough footwork, and he won't outbox Mayweahter, but he might just throw so many punches Mayweather won't do enough to win rounds
2. He is fast and southpaw which is something Mayweather has had difficulties with..
3. He is good on the inside which means Mayweather has to be sharp all times.
but he doesn't have very good defense, he was stung quite a big agaisnt Margarito who didn't land very many punches that fight, and we don't know if he can keep that high punch rate against a mover, and a guy who smothers you on the inside like Mayweather.