Blazor (https://blazor.net) is a .NET implementation of WebAssembly (https://webassembly.org/) which is a binary format supported by modern browsers.
Why Blazor: do full-stack development all in .NET.
The missing footprint of .NET is in the browser (don’t remind me Silverlight, it’s dead). Now we are able to write client applications (a.k.a. SPA), using C#, Razor and HTML with features like client side routing etc.
However at current state, Blazor does not support client side debugging yet. We have to write a lot of C# Console.WriteLine (and it will be in browser console).
This might fit for .NET backend experience developers to write client side applications.