yui/yui3 — comment on commit e1d2c90eed2334936210d1e1c6c53a43934ed517
File: src/button/css/cssbutton.css line 10
By derek:

Because without it the buttons have zero margin and "mash" together in IE 6 & 7. Which is unlike the behavior in other browsers with the current code. Considering users likely do not want that behavior, a small margin was added in.