The type of [`calc-mix()`](https://drafts.csswg.org/css-values-5/#funcdef-calc-mix) must be made consistent with the type of its [`<progress>`](https://drafts.csswg.org/css-values-5/#progress-type), which is defined with: > `<progress> = [<percentage> | <number> | <'animation-timeline'>] && [by <easing-function>]?` What is the type of `<progress>` in these examples? 1. `calc-mix(0%, 1px, 2px)` in a `<length-percentage>` context 2. `calc-mix(calc(0%), 1px, 2px)` in a `<length-percentage>` context 3. `calc-mix(--timeline, 1px, 2px)` My respective guesses: 1. `[percentage → 1]` 2. `[percentage → 1]` 3. `[]` In my opinion, the type of `calc(0%)` matching `<progress>` should always be `[percentage → 1]`, whatever the "higher" context is.