As of Chrome 51, the Cast button that would appear in the default player for videos with supported codecs and container by the Chromecast, no longer shows up. Uninstalling updates back to Chrome 50 restores this functionality.

I have a <video type="video/mp4" src="some/url">. In Chrome 50, a button appears to cast to Chromecast. In Chrome 51 it does not.

There's some news about this stuff being "improved" in Chrome 51. I see no migration path if this behaviour has been deprecated.