How does AngularJS get away with using custom HTML5 element tags and attributes?

Problem

Most browsers seem to allow you to create any element tag you like, and add any attribute name you like to elements. For example:

<!DOCTYPE html>
<html>
<body>
<div my-attribute="has no data- prefix, but seems to behave like an attribute should">
    <br/>
    <hello-world style="display:block;background:#eee">Hello Everybody</hello-world>
    <goodby-world style="background:#faa">Default display is inline</goodbye-world>
</div>
</body>
</html>

The above displays fine. It looks like undefined elements behave pretty much like spans. It also looks like AngularJS takes advantage of custom tags & attributes quite a lot - it pretty much depends on it.

But, as far as I understood, doing that sort of thing was a no-no, except in the case of custom html5 attributes with the "data-" prefix.

So, my question is: Is it no longer taboo to make up your own custom element tags and/or attribute names? Or put another way, does AngularJS rely on non-standard quirks that are not really part of the HTML5 spec, but just happen to work anyway? Did somebody at google discover a long-lost secret chapter of the html5 spec? Or am I totally misunderstanding something?

Problem courtesy of: DaveInMaine

Solution

It won't validate but it will render.

HTML5 is designed to be very forgiving (contrary to XHTML). This is in part so that "older" HTML5 browsers can allow new elements (as well as badly formatted HTML) which that version doesn't support yet without breaking the page.

This can of course be "exploited" to introduce custom tags.

That being said - you can also do this with HTML4.

As mentioned in comments: you also have the ability to prefix any attributes in an ordinary tag with data- and it will validate.

Solution courtesy of: K3N

Discussion

AngularJS don't really care, it's totally up to you. If you want your directive to be compatible with olds IE, you need to use <div data-hello-world> instead of <hello-world> and put data- in front of all your custom attributes.

HTML5 is still not 100% the norm, but it's slowly becoming the norm.

Discussion courtesy of: Benoit Tremblay

From W3C specification:

Authors must not use elements, attributes, or attribute values that are not permitted by this specification or [other applicable specifications][1], as doing so makes it significantly harder for the language to be extended in the future.

However, while the initial source of the page body (before angular processes an ng-app element) may not adhere to the W3C standards, if you use replace: true in directives, custom elements are replaced by a template HTML, which can be valid. So, in this case, you can think about an angular element as just a placeholder that is replaced with the terminal HTML output.

Discussion courtesy of: Vitalii Fedorenko

This is kinda opinionated, but so is the topic. The web is now what we make it, we have the ability to create/style/modify non-spec elements. And the Angular team may not really care about specs, its the cutting edge, they allow you to do what you want. That being said, you can do anything and everything in Angular with markup that will validate and conforms to the HTML5 spec (as in they support it both ways).

Discussion courtesy of: TheSharpieOne

This recipe can be found in it's original form on Stack Over Flow.