I wasn't familiar with HTML imports, and after searching around it looks like Firefox will never support (https://hacks.mozilla.org/2014/12/mozilla-and-web-components...). Any plans for polyfills, etc.?
I could understand if you said you need WebGL or WebAudio or some other hardware/OS interface. You don't need HTML imports.
Look, I understand how one might like HTML imports. Once you've gone down the rabbit hole of Web technology, all the fancy experimental features make it look like HTML/CSS are now adequate. They're not. It's a trap.
The real answer is writing Javascript (or something that compiles to Javascript). Always has been, always will be. Don't trust the web consortium to specify things that maybe browser vendors will implement. Do it yourself now, thank yourself later.