Actually(and accidentally) made a thread over at the NIS subforums, tried to get it moved here and what happened to it was that everyone including the mods(to move it here) ignored it.
Anyway, I’m using the latest Google Chrome(Version 28.0.1500.72 m) with the latest graphics driver (ATI CCC 13.4). Bolded the important part of the error for good measure. This is occuring on the HTML5 + NIS Beta. HTML5 Beta just redirects me to the homepage while NIS Beta works properly.
Error_game_html5 – Your browser doesn’t support all the features required by RuneScape Gold.
RuneScape uses a number of cutting-edge browser technologies, unfortunately it appears your browser doesn’t support the following features:
Web Sockets on Web Workers
There are a few simple solutions to this problem:
1. Download an up-to-date web browser. We recommend Google’s Chrome browser.
2. If you have Java installed, use our Java web client instead of the HTML5 one.
3. Install our downloadable Java client.
If problems persist, please refer to the Technical FAQs, which can be found in the Customer Support section of our website.