Blazor is a framework that allows you to code C# for frontend. Github page.
An experimental web UI framework using C#/Razor and HTML, running client-side via WebAssembly
The arrival of WebAssembly creates the possibility of building client-side web applications using languages and runtimes that are more typically used for native app development. Blazor runs .NET code in the browser via a small, portable .NET runtime called DotNetAnywhere (DNA) compiled to WebAssembly.
The programming model will be familiar to anyone who’s worked with Razor (the C#/HTML page format used by ASP.NET MVC and ASP.NET Pages).
Blazor makes your app compact and much faster since it is compiled into code the browser doesn’t have to parse.
It can run on older browsers (ex IE11) because it has polyfills.
The only note here is that it is very early. I mean, it is so early it can’t even be called alpha or anything. It is just an experiment at the moment. (Post written at 2017-08-12)