Relevant spec: https://html.spec.whatwg.org/multipage/scripting.html#selector-checked Also modifies HTMLInputElement::SetChecked to no longer modify its checked content value, instead making use of its internal checkedness state now that we can match `:checked` properly. Source-Repo: https://github.com/servo/servo Source-Revision: f18c18371d2bb5edde9d64e46b74bf01411afab3