https://drafts.csswg.org/css-text-decor-3/#text-decoration-skip-property

spaces
Skip all spacing, i.e. all characters with the Unicode White_Space property [UAX44] and all word separators, plus any adjacent letter-spacing or word-spacing.

The Ethiopic layout requirements say that an Ethiopic word space character following a word that is underlined or highlighted should also carry that underline or highlight. See https://w3c.github.io/elreq/#emphasis_with_wordspace That seems to be contravened by the current spec wording.