[css3-images] gradients and i18n

With my Internationalization hat on, I have some concerns about gradient syntax,
some of which will be easy to handle in level 4, but others maybe not:

Easy:
   - linear-gradient() directions similarly cannot be specified logically
   - radial-gradient() positions cannot be specified logically

(Easy Solution: same as 'background-position' -- add logical keywords)

Not so easy:
   - For radial-gradient(), explicit sizing is always <horizontal> <vertical>
   - For radial-gradient(), fixed-length color stops are always along the
     horizontal axis.

The last one is not just an i18n problem: using the horizontal axis is
pretty arbitrary, what if I want the vertical one?

A problem that is both i18n and non-i18n related: what if I want a circular
gradient whose radius is farthest-side along the inline (or horizontal) axis?

~fantasai