If your browser supports Media Source Extensions, YouTube's HTML5 player can use the adaptive streaming feature that's already available in the Flash player. You can also right-click the player, select "stats for nerds" and see if you can find "DASH: yes" - this means that YouTube uses adaptive streaming, slices videos and only loads the slices when they are needed. Here's a screenshot from IE11 in Windows 8.1:
"IE11 introduces support for MPEG-DASH media streaming through HTML5 Media Source Extensions (MSE). MSE extends the video and audio elements that you can dynamically change for a media stream without using plug-ins. This gives you such things as adaptive media streaming, live streaming, splicing videos, and video editing. This feature is not supported in IE11 on Windows 7," informs Microsoft.
With the introduction of Media Source Extensions and Encrypted Media Extensions, sites like Netflix or Hulu can switch to HTML5 players and no longer rely on plug-ins or separate apps. The downside for users is that it will no longer be easy to download videos from HTML5 players, since the DRM code will generate streams dynamically.