The https://www.w3.org/International/tests/repo/results/writing-mode-vertical document has not been updated in 2.5 years (browser support has probably changed since then but that needs to be checked) but it would be nice to have the link to "writing mode" right from inside the first paragraph:
These tests check whether user agents correctly apply the writing-mode property per the CSS3 spec for the vertical-lr and vertical-rl values. They are just essential tests. More detailed tests for edge cases and finer aspects of rendering can be found in the CSS test suite.