Vertical writing support has been implemented in Internet Explorer. But no other web browsers have implemented it as of spring 2007. The vertical writing support has been discussed, and will finally get supported in CSS 3, so I think this will not be an IE-only feature, but will become a web standard. See:
- https://developer.mozilla.org/en-US/docs/Web/CSS/writing-mode (note that some versions of IE uses non-standard values, although Edge appears to only understand the standards-compliant ones)
This feature is especially important in the Classical Chinese edition of Wikipedia, and many other East Asian languages such as Manchu and traditional Mongolian script. I think we can get support it now, without having to wait for Firefox, Safari, and other browsers. We can show the IE-user vertical rendering, and other browsers horizontal rendering, since even after 5 or 10 years, after CSS 3 will became completed, we still have old browsers and modern browsers to work around. Don't wait another 5 or 10 years.