There are a couple of WebM video files that appear to animate their own width and height. On Discord (as of this writing), these videos also change the size of their enclosing element. So this video of a piston, for instance, appears to squish the entire message pane!
It turns out it's well-defined!
- There are a couple of good use cases for videos with changing sizes, mostly in streaming contexts
- WebM includes the necessary subset of the Matroska media container format to support this
- The HTML specification has a paragraph for what happens when a video changes its size
- Chromium and FFmpeg FATE include at least one test case for this (phew!)