Regarding web assembly, support is limited. There's been several efforts to support WebAssembly in D but they were one man efforts that never got finished and merged to the mainline D. The only support there is in the official D compiler is for the BetterC mode, which is a minimal mode with no garbage collection, no runtime and effectively no standard library. There's been some people doing some interesting projects in it but you have to reimplement a lot of basic stuff such as data structures just to get going and at that point you might wonder why not just use C/C++ instead.

There is a fork of D compiler called OpenD which has some WebAssembly support with garbage collection and standard library. I've used it, it works for the most part, but there's bugs and if you encounter any issues then you're on your own.