[selectors] A pseudo-element should start with `::` at its start

[cdoublev]

https://drafts.csswg.org/selectors/#grammar

<pseudo-element-selector> = ':' <pseudo-class-selector>

[...]

The four Level 2 pseudo-elements (::before, ::after, ::first-line, and ::first-letter) may, for legacy reasons, be represented using the <pseudo-class-selector> grammar, with only a single ":" character at their start.

I think it should be:

<pseudo-element-selector> = ':'{1,2} <pseudo-class-selector>

Or:

<pseudo-element-selector> = '::' <pseudo-class-selector>

[...]

The four Level 2 pseudo-elements (::before, ::after, ::first-line, and ::first-letter) may, for legacy reasons, be represented using the <pseudo-class-selector> grammar, with only a single ":" character at their start.

[dlibby-]

<pseudo-class-selector> is

<pseudo-class-selector> = ':' <ident-token> |
                          ':' <function-token> <any-value> ')'

which contains the first :. So the spec already represents your second proposal.