http://jonrimmer.github.io/are-we-componentized-yet/ https://platform-status.mozilla.org/#html-imports

HTML Imports are only implemented in Chrome and it is going to stay that way, Firefox and Safari aren't having it. This is arguably a bummer but not critical for Web Components.

What is critical is that Custom Elements and Shadow DOM (V1) are in the latest Chrome and Safari but not on the horizon for Firefox and Edge. Supposedly it is planned, but unclear when it will actually arrive. 2018?