https://quirks.spec.whatwg.org/#the-percentage-height-calculation-quirk

In quirks mode, for the purpose of calculating the height of an element element, if [...] the specified value for the height property of element is a <percentage> [...] the containing block of element must be calculated using the following algorithm

It mentions <percentage>, but suspect this was written with CSS2 in mind. What should happen with a calc() that mixes a length and a percentage? https://drafts.csswg.org/css-values/#calc-func

Browsers don't agree: Blink and WebKit keep applying the quirk, while Firefox does not.

I think I prefer the Blink/WebKit behavior, it seems more self-consistent.

<!-- quirks -->
<div style="height: calc(50% + 10px); background: cyan">foo</div>

Note that as per CSS Values, calc(50% + 0px) computes to 50% so it has the quirk even in Firefox.