WPT details: https://wpt.fyi/results/css/css-mixins - at-function-cssom.tentative.html is marked tentative because the spec is currently missing a few obvious parts from the API. - The subtest "Indexed property getter" is failing due to a Bindings bug. This an existing issue (likely low impact) that's unrelated to Custom Functions. crbug.com/333965945 - dashed-function-cycles.tentative.html is marked tentative because the spec is not yet caught up with the resolution in https://github.com/w3c/csswg-drafts/issues/11500.
Custom Functions are similar to custom properties, but instead of returning a single, fixed value, they return values based on other custom properties, parameters, and conditionals.
Explainers: https://css.oddbird.net/sasslike/mixins-functions