[css3-background] [css3-box] [css3-ui] box-sizing keywords

The 'box-sizing' property takes two values, 'border-box' and 'content-box'.
    http://www.w3.org/TR/css3-ui/#box-sizing

The 'background-origin' property takes three values, 'border',
'padding', and 'content'.
    http://www.w3.org/TR/css3-background/#the-background-origin

They're all referring to the same set of boxes. IMHO, they should
be using a consistent set of names.

I propose adopting the -box nomenclature, as it's more explicit
about what it actually means.

~fantasai