Encrypted Media Extensions
Netflix supports HTML5 video using EME with a supported web browser: Chrome, Firefox, Microsoft Edge, Internet Explorer (on Windows 8.1 or newer), or Safari (on OS X Yosemite or newer). YouTube supports the HTML5 MSE. Available players supporting MPEG-DASH using the HTML5 MSE and EME are NexPlayer, THEOplayer by OpenTelly, the bitdash MPEG-DASH player, dash.js by DASH-IF or rx-player.
Use of Ogg formats in HTML5
CTO at Opera Software, Håkon Wium Lie explained in a Google tech talk entitled "The  element" the proposal of Theora as the video format for HTML5:
After the presentation, Lie was asked whether Opera will support other formats than Ogg: