[css-pseudo-4] Add a note explaining Element.pseudo() behaviour #12159

[danielsakhapov]

As per resolution in #12159, Element.pseudo() returns CSSPseudoElement even when invoked on Element that can't generate the requested type.

[astearns]

The note is good to explain and reference the reasoning why, but notes are non-normative. We’ll also need a normative change to the algorithm

[danielsakhapov]

The algorithm will be unchanged, won't it? It still returns a CSSPseudoElement object if the selector is valid.
I started the issue and planned this note as just an additional info to clarify the case like e.g. <input>.pseudo(::before) returns CSSPseudoElement even though <input> can't generate ::before:.
My point is that per current algorithm description says: the selector ::before is valid, so it returns CSSPseudoElement. I don't see what changes we should add with that resolution, could you, please, explain?

[danielsakhapov]

@astearns gentle ping