I'm sure there are some extremely design-heavy sites that do it, but by then you would be fighting the DOM/CSS pretty hard, as default behaviors for stuff like

I'm sure there are some extremely design-heavy sites that do it, but by then you would be fighting the DOM/CSS pretty hard, as default behaviors for stuff like underline position and 縦中横 (short bursts of horiz text within vertical text, for things like two-digit numbers) seem to be sketchy at best, and many things still don't seem to be uniform across browsers. UX would be affected as well, as things like text selection or right-clicking would feel quite awkward as it is so exceedingly rare. While us Japanese are fully capable of reading both horizontally+LtR and vertically+RtL, it's not in our expectations for the latter to appear in online web text.