Raw source text of the argument of a functional pseudo-class or
pseudo-element, from funcPosition (the position of its name) to the
closing parenthesis the tokenizer is left on by
skipPseudoFunctionContents.
Parameters
funcPosition: number
Returns string
readLayerName
readLayerName():string[]
Reads a <layer-name> (<ident> [ '.' <ident> ]*) at the current position
and returns its parts, or null if there is none.
Raw source text of the argument of a functional pseudo-class or pseudo-element, from
funcPosition(the position of its name) to the closing parenthesis the tokenizer is left on byskipPseudoFunctionContents.