https://www.w3.org/TR/css-text-3/#word-spacing-property says "General punctuation and fixed-width spaces (such as U+3000 and U+2000 through U+200A) are not considered word-separator characters."

U+205F Medium Mathematical Space is not mentioned and so it is not clear if it is considered a word-separator character. I'm guessing not. It should be mentioned to make it clear.