I don't mean HTML text nodes, I mean still the single big SVG like they do now, but with SVG <text> elements instead of <path> elements. They do know (I suppose) how much space that element would take since they're asking HarfBuzz to tell them.
why not? if you control the font and the font size then can't you have harfbuzz tell you how wide each glyph is going to be, and it'll fit precisely? (assuming the browser also uses harfbuzz for this, which it does)