[css-color-6] `contrast-color()` doesn't include the existing syntax from `css-color-5`

[romainmenke]

https://drafts.csswg.org/css-color-6/#colorcontrast

<contrast-color()> = contrast-color(
  [ [ <color> && [ tbd-fg | tbd-bg ] && <target-contrast>? ] |
    [ <color> && [ tbd-fg | tbd-bg ] && <target-contrast>, <color># ] ] )
<target-contrast> = <wcag2>

https://drafts.csswg.org/css-color-5/#funcdef-contrast-color

contrast-color() = contrast-color( <color> )

---

In css-color-5 there is already a minimal version of this function.
The definition in css-color-6 should allow passing only a single color argument.

[svgeesus]

@LeaVerou any problem to just add the max keyword to css color 6 contrast-color()?

[svgeesus]

Ah, I see the definition of contrast-color() in CSS Color 5 no longer has the max value, so this issue can be closed

[romainmenke]

The same issue still exists, although for a slightly different syntax.
The syntax from css-color-5 isn't valid/covered in css-color-6.

I've updated the description above

[svgeesus]

From Computing a Contrasting Color: the contrast-color() function:

The only mandatory argument is the base color against which the contrast is computed. This is typically (but not necessarily) a background color.

and

The <target-contrast. argument specifies the contrast algorithm(s) to use. If no color candidates have been provided, <target-contrast. may be omitted, in which case a UA-chosen algorithm is used.

That seems to be exactly the syntax in Color 5: one color, no candidates, no target contrast.

[romainmenke]

https://drafts.csswg.org/css-values-4/#comb-all

A double ampersand (&&) separates two or more components, all of which must occur, in any order.

The issue is && [ tbd-fg | tbd-bg ] which is mandatory in the syntax definition.

So contrast-color(red tbd-fg) is considered valid, while contrast-color(red) isn't.

[svgeesus]

Huh. So the grammar and the prose don't match. @LeaVerou is it intended to make [ tbd-fg | tbd-bg ] mandatory?