[css-color-hdr] Require H2 >= H1?

The proposal implies, but does not explicitly state, that H2 > H1. In particular, H1 is used when H <= H1 and H2 when H >= H2.

I suggest that we add prose explicitly stating that H2 >= H1 (equals is a pointless no-op, but easy to specify) and then state what happens if this is not true.

[css-color-hdr] Consistently use log (stops) definition of headroom, …