It looks like your using a web browser without WebGL.
WebGL is a technology that allows users to render graphics through the GPU from their web-browser.
Writing a tectonics simulator in a browser is hard stuff. We're going to need all the tools we can get.
It looks like you're using a browser that supports WebGL, but you just need to modify some settings, first. Here is a link that will help you get started.
Until then, here's a video of what you're missing: