These browsers usually have reasons to have their own styles. Sometimes they are there to be consistent with the base OS (drop downs, buttons, etc) or they are there to best suit the format that the browser runs on small screen (phones), large screen (desktop) etc or to take care of device input types (touch screen devices would have larger buttons by default whereas pointing device doesn't need to have large buttons). I think it's best to let the browser decide what it wants to decide and build our app around browser differences instead of forcing our own design down browser's throat.
I think the problem is manly IE vs Chrome vs Firefox.. (and of course Opera). Example: Why would you want to have by default a padding of 10px in IE, a padding of 5px in Chrome, 7px in Firefox and 0px in Opera? Of course is easier to make a default for all those browsers to 0px or whatever you want.
