ogv.js works pretty well on faster iOS devices in Safari and apps that use the JIT-enabled WKWebView, but in Chrome and other apps using the old UIWebView control it's too slow to play back even low-res video.
However, iOS actually does support a free-software-compatible format: AVI container with MJPEG video and PCM audio.
This is a horrible format for many reasons:
* MJPEG has no colorspace standards
* MJPEG is inefficient, so to get the bitrate down must decimate the framerate and it still looks bad
* PCM is inefficient, so to get the bitrate down must decimate the sampling rate and bit depth, so it sounds bad
* AVI sux lol!
But it can be easily created from avconv, and it works in a <video> element. YAY
Something like this seems to work:
avconv -i Sneak_Preview_-_Wikipedia_VisualEditor.webm -filter:v scale=640:360: -c:v mjpeg -b 512k -r 5 -ac 1 -ar 22050 -c:a pcm_u8 preview-u8-b512.avi
sample at https://brionv.com/misc/mjpeg/
Note this AVI won't load in Safari on desktop! Only works on iOS. :P