the line-bounded attribute mask replaced the browser reading instead of adding to it so an attribute value carrying a greater-than and a line break hides the heading opener

the line-bounded attribute mask replaced the browser reading instead of adding to it so an attribute value carrying a greater-than and a line break hides the heading opener

Found by the round-10 adversarial security review of build/itd-183, and fixed in the same round. REGRESSION INTRODUCED BY THIS ROUND'S OWN FIRST COMMIT.

Closing the runaway attribute mask by bounding its value to its own line REPLACED the reading a browser's tokenizer takes -- the closing quote is the next matching quote ahead -- rather than adding to it. The two readings disagree about exactly one shape, and the round deleted the reading that resolves it:

<h2 title="a>
b">Audit Notes</h2>

Every renderer reads that as one h2 whose title attribute is a>\nb and whose text is Audit Notes. The line-bounded mask declines it, because the value does not close on its own line; and the unmasked reading cannot parse the opener at all, because rawHeadingOpenRe's [^>]* stops at the > INSIDE the value, so the title read is b">Audit Notes and slugs to something else. The round-9 mask, unbounded, blanked that > and refused.

Verified leaking for the single-quoted form, for a role="heading" element, for a harmless attribute ahead of the spanning one, and for a trailing inline element carrying the break.

Remedy, and the doctrine the round already states: a mask may ADD a reading and may never take one away. Both maskings are taken alongside the unmasked text and a heading is refused if any reading names an excluded one. The conservative reading is load-bearing in the other direction, so neither may be dropped: a stray quote earlier in the file pairs, under the browser reading, with the opening quote of a LATER legitimate value, and that value is then never masked.

LESSON, and it is the branch's own: substituting one reading for another is the move that has opened a leak in five consecutive rounds. Adding is safe; replacing is not.