Right now there’s a long section for mobile-first, then a long section for browsers with CSS Grid support. (Prior to using a preprocessor, this made it easy to disable the grid rules for testing.)

It would be better to have all the rules for each thing right next to each other. Grid-related rules can be enclosed in a form that surrounds them with the correct media query, and a parameter can turn these rules on or off.

Also some of the longer comment blocks need re-wrapping.