[css-values] add font-size relative pixel unit rpx

I came up with an idea of a unit that basically behaves like px when user's font-size is as same as browser's default, but it inflates and shrinks when user changed the font-size. I'd like to name this unit rpx.

the actual unit size would be calculated like this:

1rpx = 1px * (user-set font-size) / (browser's default font-size)

It allows us pixel-accurate design when needed while being responsive when user prefers different font-size. It will successfully replaces the html {font-size: 62.5%} hack, which results in changing unit size in simular fashion, as described in this SOF answer.

Start by reviewing the CSS Values issue and the proposed rpx formula in the report. Determine whether the CSS Working Group accepts the unit and what specification details must be resolved. Done means reaching agreement on the unit's behavior and documenting the accepted definition in the relevant CSS Values work.