The declarative style that Angular coaxes you into makes things incredibly easy to understand - you can write code like
I can also do simple directives like <loading-spinner></loading-spinner> and create a nice template for CSS to hook into by using the loading-spinner element tag without having to distract from the rest of my DOM with a nesting of 8 divs that can easily be tucked away in a reusable template.
The reason being, considering that HTML5 is an ever evolving spec that it might be possible for a new element to be introduced that could be using the same name as one of your examples. That might muck things up a bit. At least visually, as I would imagine the directives would likely continue working the same as before. It's just that the "they must be directives" logic would fail in that instance.
Interestingly, out of the box Angular provides directives as a, input, or form—meaning that when you write <input> you are, in fact, invoking a directive that provides extra features along with native tag's functionality. While using Angular, you may as well consider every HTML element a directive.
Nevertheless, Angular uses ng- prefix with their built-in directives that don't replicate some existing tag's functionality closely.
 -- http://docs.angularjs.org/guide/directive
The only requirement is that they have a dash in the name. So while <sidebar> is not a valid HTML5 element, <a-sidebar> would be.
Yes, this is actually the recommended practice from the documentation. Built-in Angular directives are typically prefixed with `ng-`, and they suggest that other projects/developers use their own prefixes to avoid conflict.
Why not use XHTML at that point? If XHTML ever had a clear mission statement in life it was that.
I mean I thought XHTML was a curse a few years ago when it was The Thing Every Good Designer Uses™ since it couldn't be rendered as XML in the most popular browser. But hopefully that's not as much an issue anymore. Or maybe it is. I haven't kept up with IE.
Or you know, an adoption of only the needed part, instead of all the BS junk that comes with XML namespaces.
>Why not use XHTML at that point?
Because of all the other junk that comes with it. And because it comes from W3C.
When users gain the ability to define their own tags, you might not know which ones are driven by directives, the custom element, or both even.