If there is a Chromecast around and:
After searching the Chromium source code, I figured out how to do it:
video::-internal-media-controls-overlay-cast-button {
display: none;
}
Example: http://jsfiddle.net/f1quhd2L/8/
There is another options to disable casting button, this option actually disables the button instead of hiding it.
<video disableRemotePlayback src="...">
https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/disableRemotePlayback https://developers.google.com/web/updates/2015/11/presentation-api