This change makes Ladybird correctly handle all “encapsulation” tests in
the https://wpt.fyi/results/accname/name/comp_host_language_label.html
set of tests in WPT.
Those all test the requirement that when computing the accessible name
for a <label>-ed form control, then any content (text content or
attribute values) from the control itself that would otherwise be
included in the accessible-name computation for it ancestor <label> must
instead be skipped and not included.
The HTML-AAM spec seems to try to achieve that result by expressing
specific steps for each particular type of form control. But what all
that reduces/optimizes/simplifies down to is just, “skip over self”.
Otherwise, without this change, Ladybird includes that “self” content
from those “encapsulated” elements when doing accessible-name
computation for the elements — which results in AT users hearing
unexpected extra content in the accessible names for those elements.
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