I don't know anything about how they're doing this, but I wonder if it has something to do with the bytecode abstraction used in D3D. In D3D, you compile HLSL to bytecode which then gets passed down to the kernel, but in the OGL world you pass source straight down to the kernel. Definitely still risks there, but significantly diminished, and some simple verification in userland would make it next to impossible to get a lot of nasty code down into the kernel.
MS could (and probably would) write an Angle style abstraction layer that runs WebGL on top of D3D9 or D3D11 anyway, so I think source going to the kernel isn't a big deal.
It's possible that because MS has more knowledge of the graphics drivers work in Windows they know about some dangerous security holes that Mozilla, Google, or even Nvidia or AMD aren't aware of, but its equally possible that they just don't want to support WebGL for political reasons and this is a semi-technical excuse.
The D3D bytecode IR generated by the HLSL shader compiler in the runtime isn't verified for security concerns before being passed to the driver, so there's no real extra protection there, so ultimately D3D makes it no harder to lock the GPU than OpenGL (ES in the WebGL case) does.
It is a completely separate house / separate bed issue.
You do not have permission to call Direct3D directly. You can't even do cool hacks like you can in WPF, stealing the Direct3D video feed and writing it to a movie file. Everything is abstracted away by (underpowered) APIs.
All Secunia advisories on .NET Framework / Silverlight are presently patched, and the total number is relatively small compared to other technologies like web browsers and Flash.
I don't really know enough about WebGL to compare, though.
That blog is trolling. The blog author copied and pasted the material from the blog we are currently discussing, and replaced WebGL with Silverlight.
Silverlight is more secure than WPF, too, by the way, and has to be. For example, in WPF there is a very insecure static method that allows you to steal a bitmap of the entire screen! This was one of the first things taken out of Silverlight.
If you want to know more about Silverlight security, ask Nick Kramer who maintains the Silverlight security best practices document for Microsoft.
It's the same thing as Safari, IE, and Chrome using the GPU to composite pages: Silverlight is hardware accelerated, but does not expose the hardware acceleration primitives. GL or Direct3D expose more-or-less direct hardware access, which is very different.
"With the release of Silverlight 3 Beta 1 GPU (Graphics Processing Unit) acceleration (or hardware acceleration) is now available."
I'd like to hear what could make that secure that couldn't be used with WebGL.