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UE4 is a free 3D game engine and editor by Epic Games. Unreal Engine 4 allowed HTML5 game creation earlier, and since version 4.16 it provides an extended support for HTML5, including features like WebGL2 and WebAssembly (aka wasm). This brings native 3D experience to modern web games.
SpecsUpdate
Pros
Pro Visual Scripting
Unreal Engine's visual language aka blueprints allows programmers to write games almost without limits, meaning that everything you would do in c++ is possible in blueprints too. Even if you find missing functions in blueprints, you can extend this language via plugins.
Blueprints are beginner-friendly and suitable even for non-programmers.
Pro (Almost) Free and open source
You don't need to pay anything unless your project earns $3 000 or more. Starting from that point, you will have to pay 5% royalty to Epic Games.
The source code is licensed under UnrealEngine's EULA