There's a WPT testing that text-decoration-thickness takes calc() mixing percentages and lengths.
https://github.com/web-platform-tests/wpt/blob/b5c73fa21b/css/css-text-decor/text-decoration-thickness-valid.html#L24-L25

Except the spec specifies the syntax to be: auto | from-font | <length> | <percentage>, which means that they can't actually be mixed? If they are allowed to be mixed, the type should probably be <length-percentage>.

It seems like the mixing is supported in Firefox / Chrome. Safari doesn't currently support percentages.

cc @kojiishi @fantasai @shivamidow