>>Check this:
>>http://stackoverflow.com/questions/4767010/why-does-css-padding-increase-size-of-element
>
>Thanks, this resolves the issue.
>
>I remember having had to deal with this a few years ago when moving to Html 5. I just forgot about it.

We've supported "box-sizing: content-box;" for sooo long (CSS2)!
Viva "box-sizing: border-box;"