OpenFin leverages Chromium to support video playback. The Chromium team has documented the supported codecs in the following article: https://www.chromium.org/audio-video
Some further reading on the design decision for codecs can be found here: https://blog.chromium.org/2011/01/html-video-codec-support-in-chrome.html
H.264 Codec Support
Chromium does not support the H.264 codec as the team favors open libraries.