What is this notation and why does it work in Chrome?

Question

I've just tried this:

<style>
  rgraph:option {
    background: red;
    padding: 3px;
  }
</style>

<rgraph:option>canvas</rgraph:option>

And in Chrome (69.0.3497.100) it worked. But not IE 11 or Firefox.

And what is it? Obviously some new CSS feature that's implemented in Chrome only.

I do like the idea of being able to use custom tag names.