Runescape 3 is the name for the upcoming version of the Runescape client (the part of an MMO that runs on the player's computer). Just like the current version (called "Runescape 2" during development) was a major technological and visual change from the original version (pesudo-3D using 2D sprites as some of you may remember), Runescape 3 promises to do the same (and will likely just be called "Runescape" like always, with the older versions getting re-named to note that they're the old ones).
Built in HTML5 instead of Java, the beta testing version of the client (which is already open to some members) only runs in Chrome right now, but should run in more browsers as proper HTML5 support spreads. The change in platform and re-write of the engine allows many things that were difficult, impossible, or simply too slow in the old engine. Increased draw distances, skyboxes, weather effects, controller support, and more. Of course moving to HTML5 brings up the possibility of being able to play Runescape on something like a smartphone or a tablet, though this isn't currently a goal for Jagex.
In addition to the info about the Runescape 3 client, information about in-game updates such as a new skill (something that usually only happens once every few years), new graphics, and a customizable UI was released to the press. Of course, if you only watch the video to see British people cosplaying and dressing up a cathedral and castle about a videogame as part of their job, I don't blame you.
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