The `TruncateHTMLParser` used `deque.remove()` to remove tags from the
stack when processing end tags. With crafted input containing many
unmatched end tags, this caused repeated full scans of the tag stack,
leading to quadratic time complexity.
The fix uses LIFO semantics, only removing a tag from the stack when it
matches the most recently opened tag. This avoids linear scans for
unmatched end tags and reduces complexity to linear time.
Refs #30686 and 6ee37ada32.
Thanks Seokchan Yoon for the report, and Jake Howard and Jacob Walls for
reviews.
Backport of a33540b3e2 from main.
9f2ada875b