[multi][editorial] Rewrite syntax with implicitly optional comma

[cdoublev]

Eg. rewrite [<foo>,]? <bar> as <foo>? , <bar>.

In case there is any doubt for [<bg-layer># ,]? <final-bg-layer>.

the # and ? multipliers may be stacked as #?

https://drafts.csswg.org/css-values-4/#component-multipliers

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edit

Hmm, actually there is currently a mismatch between...:

<step-easing-function> = step-start | step-end | steps(<integer>[, <step-position>]?)

... and:

steps(<integer>, <step-position>?): The first parameter specifies the number of intervals [...]

This confuses w3c/reffy, which extracts the latter whereas some contextual definitions like the latter may use an abstract syntax, eg. pow(A, B) instead of the formal syntax pow(<calc-sum>, <calc-sum>).

Do you mind if I replace steps(<integer>[, <step-position>]?) with <steps()> in this PR or another one?