Highly recommended for CSS Grid is Jen Simmons Mondrian with auto flow. https://labs.jensimmons.com/2017/01-011.html. Try 'C' and 'D' then change the window size. Also the youtube video that goes along https://www.youtube.com/watch?v=qNtJ5p3h2A4.

Just very cool.

The one thing about auto flow I wish I understood was how to detect, and fill, gaps. If I have some random collection of differently sized objects, how do I ensure that auto flow leaves no gaps? Or even detect that so I can add something?