http://dev.w3.org/html5/webvtt/#webvtt-vertical-text-cue-setting
```
A WebVTT vertical text cue setting configures the cue to use vertical text layout rather than horizontal text layout. Vertical text layout is sometimes used in Japanese, for example. The default is horiontal layout.
```
The vertical keyword specifies both layout and direction. And it seems to be the only way to specify a direction at all, IIRC.