Improve :focus-visible tests

- Add explanatory text to each one
- Make the manual test path accessible to color blind testers
- Bring headers up to date
- Create a consistent style for :focus-visible and :focus:not(:focus-visible) for easier understanding of test results

Bug: 817199
Change-Id: I97191d5c8395576a42c923a025d05d2d61e79552
Reviewed-on: https://chromium-review.googlesource.com/1093697
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Jonathon Kereliuk <kereliuk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567266}
8 files changed