This adds (or at least stubs-out) the following:
- display-mode
- dynamic-range
- environment-blending
- forced-colors
- horizontal-viewport-segments
- vertical-viewport-segments
- inverted-colors
- nav-controls
- prefers-contrast
- prefers-reduced-data
- prefers-reduced-motion
- prefers-reduced-transparency
- scripting
- video-color-gamut
- video-dynamic-range
The `@media (inverted-colors)` CSS that the spec requires we add to the
UA style sheet does not actually do anything for us yet since we don't
support `filter`, but it seemed sensible to include it now to avoid
forgetting later. :^)