[css-values-5] Clarify how <attr-unit> should be parsed #12479
Description
The spec for this says:
If given as an
<attr-unit>value, the value is first parsed as iftype(<number>)was specified, then the resulting numeric value is turned into a dimension with the corresponding unit, or a percentage if % was given. Values that fail to parse as a<number>trigger fallback.
<number> can mean a literal number, but it can also be a math function, or various other functions that produce <number>. Chromium added a crash test which checks that a non-literal appearing doesn't break the browser:
<!DOCTYPE html>
<link rel="help" href="https://crbug.com/405422528">
<div data-crash="sibling-index()" style="width: attr(data-crash px)"></div>
<div data-crash="sign(1em - 1px)" style="width: attr(data-crash px)"></div>But their fix assumes that only <number> literals are allowed.
The spec is ambiguous here. Only allowing literals is one interpretation (and certainly the easiest to implement!) but the other interpretation (which I read the spec as meaning) is that this:
<div data-crash="sign(1em - 1px)" style="width: attr(data-crash px)"></div>...should produce a width value equivalent to calc(sign(1em - 1px) * 1px).
cc: @lilles