Really nice idea.

One thing that's adding to the "frustration factor" of the CSS problems is that the updates seem rather racy (Firefox 63, MacOS), in the sense that if you edit further while an update is already running, it doesn't seem to trigger a further update. Makes moving things around 1px at a time more annoying that it should be.

(I'm another one stuck with a just few pixels difference on the triangle for CSS Puzzle 7. Once someone's claimed the final prize, might there be an option to skip through to the later puzzles?)