(no bug) fix typo in a comment within a reftest. DONTBUILD
No bug - Fix copy/paste error in 'mix-blend-mode' comment. (DONTBUILD)
bug 910506 - update harfbuzz to upstream release 0.9.21 plus latest bugfixes (commit 3d2c4f0c2ff8fab4262988aad65b170e5b479b20). r=jdaggett