The webapp includes a copy of the documentation for Rust, Go, Nim, Lua, Julia, Haxe, Tensorflow, Numpy, among many other technologies. If you consider these "web dev" then label me crazy because I see these as system programming languages and/or data science libraries more than anything else. The lack of Scala, C#, etc is due to the complexity of the documentation which cannot be downloaded so easily as the others, or the license of the documentation itself. Feel free to contribute through their repository [1]

Java is under the JDK section by the way.

[1] https://github.com/Thibaut/devdocs