Empty or non-matching values for ID reference and ID reference list - Clarification on author responsibilities

[giacomo-petri]

Closes: #2225

Clarification on author responsibilities concerning empty or invalid references for properties accepting ID reference (or ID reference list) as values.

The intention is, considering the dynamic nature of contemporary websites, to allow authors to set empty or "invalid" values, acknowledging the DOM can be populated accordingly when necessary.

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