https://drafts.csswg.org/css-text-4/#valdef-text-spacing-trim-space-first

Set fullwidth opening punctuation with full-width glyphs (spaced) on the first line the block container. Otherwise as trim-auto (unless allow-end is also specified).

I think this should be:

Set fullwidth opening punctuation with full-width glyphs (spaced) on the first line of the block container and each line after a forced line break. Otherwise as trim-auto (unless allow-end is also specified).

"and each line after a forced line break" is necessary in this definition because the space-first value exists for compat requirements for existing content, where often <br> is used to separate paragraphs. See: #9511 (comment)