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

Question

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

<         


        

Answer 1

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.