"CSS Rhythmic Sizing" is the spec. of vertical rhythm. Vertical rhythm is a principle of typography. It is about spacing text. More generally, it is about vertical stacked elements. We can create a more comfortable layout with vertical rhythm. We can imagine that we're put all elements on a sheet of lined papaer. The sizes of elements is according to the size between the lines.
Oh, great, thank you for working on this. Please also have a look at tests in: https://github.com/w3c/csswg-test/tree/master/css-rhythm-1
Hi Koji, there is the test result here. And it is not completed with some bugs in my patch. : https://test.csswg.org/harness/results/css-rhythm-1_dev/grouped/