Improve accessibility

[waldoj]

This scores well on an a11y test, but there’s more to be done. We’ve got color-blindness covered (#29), but there are other things, including making the header nesting logical and h1-rooted and using semantic tags for emphasis etc..

[waldoj]

There's more to be done with this new home page color / grading scheme:

Sure, the letters are a fallback for the color blind, but it'd be better to select a color palette that also works for dichromats. With the correct selection of green, light green, yellow, orange, and red, those can also form an intensity scale that's meaningful to dichromats. (Or so I speculate. I've never tried.)