We have an issue with Live2D in the WebGL build process. We tracked it down to Live2D after several weeks and it happens even in an empty project with only Live2D sample itself. Happens in Unity 2019.4, 2020.3 and possibly all others (tested using both latest and last year's SDKs)
Basically when building using Command Line ( https://docs.unity3d.com/Manual/CommandLineArguments.html
) Emscripten throws "[Error 6] The handle is invalid" and it breaks the build. Building from Editor works fine, however we require command line builds to set up the remote building process.
The basic Emscripten build log doesn't mention Live2d explicitly but it is the only asset within the project, and if you set Emscripten compilation flags to DEBUG it will show Live2d being the culprit in the linking stage. You can find the non-debug build log here: https://imdev.pl/share/d846b9d7-0327-48ce-b17f-ed44009fbffa/BuildWebGL.txt
I believe that this error might be related to this showing up during the build - I'm not sure what this is doing or what is launching it, but it is an additional process and [Error 6] is related to spawning multiple processes during build. Maybe this can just be removed and it will fix the issue?
Here is the minimal project we used to check that it is indeed an issue with Live2d - https://imdev.pl/share/d36110cb-a13c-43e0-b51e-05cc8e05f374/Live2dCLITest.zip
Run it by using the following command in the command line (change project and Unity paths accordingly)
d:\Work\Unity\2020.3.3f1\Editor\Unity.exe -quit -nographics -batchmode -buildTarget WebGL -projectPath d:\Work\Live2dCLITest -logFile D:\BuildWebGL.txt -executeMethod BuildSystem.Build_WebGL