Look above the letters. Every one of them carries a dot. This is the puncta extraordinaria — the extraordinary points — a scribal mark older than the vowels and the accents printed with it, copied faithfully for centuries by scribes who did not record what it was for.

The dotted word is הֵמָּה, the independent pronoun they, standing in apposition to their witnesses. Hebrew does not need it: the two verbs that follow already carry their subject, so the pronoun is emphatic rather than required — and it is the only dotted word in the verse. Readers have joined those two facts and proposed that the dots mark the pronoun as questionable, or as an addition. That is an explanation offered long after the mark was already ancient, and so is every other explanation of these dots. Nobody knows.

There are fifteen dotted sites in the whole Hebrew Bible — counted out of the underlying data across all four volumes, not recalled. Ten are in the Torah: Genesis 16:5, 18:9, 19:33, 33:4, and 37:12; Numbers 3:39, 9:10, 21:30, and 29:15; and Deuteronomy 29:29. Five are outside it: 2 Samuel 19:19, Psalm 27:13 (which carries a lower dot as well as upper ones), this verse, and Ezekiel 41:20 and 46:22. This is the only one in Isaiah.

Nothing about the letters is unusual. The word is spelled normally, pointed normally, accented normally, and read normally, so the reading line is unchanged — there is nothing to change.

A word on why this is the only note in Isaiah. It is not an oversight, and it is not a judgment that nothing else in sixty-six chapters wants explaining. The dots are different in kind from a difficult word: they are visible. They sit on the page in plain sight, and a reader who notices them and finds no explanation has fair reason to think his display is broken. Not leaving a visible mark unexplained is a narrower duty than explaining every difficulty, and it is the one being discharged here.