What is WebAssembly?
WebAssembly (Wasm) is a new binary format language for a stack-based virtual machine that is supported in modern browsers (Firefox, Chrome, Safari, and Edge). A large part of the appeal is that it's a portable compilation target for other high-level languages like C, C++/, and C#. It aims to be fast (at near native speeds), memory safe and sandboxed since it executes within a VM, and an open standard for others to use.
Why use WebAssembly?
Just within the .NET community there are currently 3 major technologies leveraging Wasm including Blazor, Uno, and Ooui. Each of these have their own niche,and they will interest developers for different reasons.
Blazor is an experimental .NET framework that originates from the ASP.NET team that allows you to create a web application using C# and Razor syntax. In some ways it's almost like a replacement for some web frameworks (like Angular) that are ASP.NET Core centric. It's likely to appeal to developers who have used ASP.NET Web Forms and MVC (or dare I say it… Silverlight).
Blazor has a much gentler learning curve for those groups compared to learning Angular or React. Also, since it's Wasm-based, it can run fully in the browser. This technology is backed by Microsoft and is actively being developed by the ASP.NET Team. It's open-source, free, and uses .NET Standard. While still experimental, it's likely to have the brightest future of the .NET Wasm technologies and it will ship as part of ASP.NET Core 3.0.
Uno is an interesting cross-platform solution that allows you to write C# and XAML code for mobile and web using a free UWP bridge. Uno's technology leverages both Mono (for mobile) and Wasm for web. Apps can be written in a similar style to UWP XAML, and your XAML is parsed to C# and further compiled into Wasm. It doesn't require much from a XAML developer, as it allows them to reuse many of their existing skills for databinding, control and data templating, MVVM, etc. There is an online demo available that gives you some idea of how this works that lets you modify XAML in browser to configure a selection of UI controls. The technology is still early, but it's undeniably cool tech and should appeal to desktop XAML developers.
Ooui is another cross-platform .NET UI library that enables Wasm using XAML and C#, though in this case it's using Xamarin.Forms as its base. The specific appeal of this library is that it can be used with Xamarin.Forms (which is code that was already explicitly cross-platform), and it enables existing Xamarin apps to potentially be extended to web. This project is also relatively early, but it has a number of samples that illustrate its possibilities.
There are some missing features eventually coming to Wasm that will make it more compelling to developers. Low-level pthread style threading and shared memory haven't been added yet, though they are on the roadmap. Garbage collection is another huge feature that many C# developers have come to expect since it makes memory management much easier. Not many enjoy going back to C style memory allocation and deallocation.
There are many other import features listed on the features roadmap for post MVP (minimum viable product) like Exception handling and SIMD support. WebAssembly 1.0 was their MVP release and since we're now in the post MVP era, many of these features are being actively worked on. Supporting these features would enable a more complicated class of heavyweight application development which Wasm would be uniquely qualified for (provided they're able to ship these features in a timely fashion).
Though WebAssembly is several years old now we're still in the early stage of its existence, and it's not immediately obvious how it will evolve in the future. It's likely to be popular though (it's already a big conversation topic), and new projects will likely start using it in the coming year. Though we've focused on the .NET side of Wasm, it's likely other languages use it too. Rust, Java… really anything is possible provided developers are dedicated enough to build the tooling for it. At this stage it's hard to predict which flavors of WebAssembly platforms will be the most popular, but it seems very likely that many developers will take advantage of it in the future.