This are 2 nice blogs to get in context:
But taking a step back, Anders is talking a lot about the c# compiler as a service, imagine you can use c# directly so you can use it like HTML/C#, you can access all the DOM objects with c# and the browser (IE12) integrates this compiler service so your C# could be compiled on demand. In theory, you could use the Razor engine and the syntax, if you want to use C# in the server use the @ token, if you want C# in the client you use another token, and with this, some part of the page is created in the server and in the client is compiled by the browser.
Maybe what is needed is a JS# language, that is optimized for this context so C# don't get polluted. But this new language has to be a subset of C#, so we can enjoy all the awesomens of .Net (the .Net API is a joy to use and of course the Tooling)
I don't know if I am making any sense, I'm on my sleeping pill.