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.