[css-paint-api] Allow inputArguments define optional arguments

[zheeeng]

The custom paint requires a color argument.

class MyPaint {
    static get inputArguments() { return ['<color>']; }

    paint(ctx, size, styleMap) {
        // Paint code goes here.
    }
}

registerPaint('my-paint', MyPaint)

At present, the correct usage is background: paint(my-paint, red).

Wish there is a way to make this color argument be optional: background: paint(my-paint)

[css-meeting-bot]

The Working Group just discussed optional arguments in paint functions, and agreed to the following:

• RESOLVED: Deferred to L2

The full IRC log of that discussion

<fantasai> Topic: optional arguments in paint functions
<TabAtkins> github: https://github.com//issues/763
<dbaron> github: https://github.com//issues/763
<fantasai> TabAtkins: Somebody wants optional arguments in paint functions
<fantasai> TabAtkins: It's a very reasonable request
<fantasai> TabAtkins: However, this falls under the more general issue of optional bits of syntax in custom properties
<fantasai> TabAtkins: which is currently not allowed because of issues around parsing ambiguity
<fantasai> TabAtkins: So we need disambiguation support first
<fantasai> florian: Is this a hard problem, or we need to sovle it but haven't yet?
<fantasai> TabAtkins: latter
<fantasai> TabAtkins: Definitely not impossible
<fantasai> TabAtkins: Support for more syntaxy things is slated for L2
<fantasai> fantasai: So proposal is close deferred to next level?
<fantasai> TabAtkins: That's what I would like
<fantasai> Rossen: Any other opinions?
<fantasai> RESOLVED: Deferred to L2
<fantasai> fantasai: Is there anything else is needed for the spec?
<fantasai> iank_: No
<fantasai> TabAtkins: TypedOM has changed how it handles image values
<fantasai> TabAtkins: which might affect this, but it's not affecting this spec