For whatever reasons (mostly intertia, I would imagine), the languages which are most associated with vertical text, Chinese and Japanese, are written horizontally on the web.

But vertical text may be becoming a reality down the road.

At Unicode.org, there is an interesting illustrated pdf UTN #22: Robust Vertical Text Layout. It goes into gruesome detail about how vertical text can be arranged, how scripts are combined, and so on.

The consequences for CSS and the web are pretty broad. For one thing, it will allow content in the Yi and Mongolian scripts to make their way onto the web for the first time.

Another possibility is that we will see a resurgence in vertical text in the well-known languages Chinese, Japanese, Korean.

(Thanks to Brion VIBBER for pointing this out to me.)

A mockup of a hypothetical traditional Mongolian Wikipedia by Node_ue at Wikimedia Commons (no, that’s not sideways ☺)