Are custom elements valid HTML5?

Question

I\'ve been unable to find a definitive answer to whether custom tags are valid in HTML5, like this:

Hello!

Answer 1

I would like to point out that the word "valid" can have two different meanings in this context, either of which is potentially, um, valid.

1. Should an HTML document with custom tags be considered valid HTML5? The answer to this is clearly "no." The spec lists exactly what tags are valid in what contexts. This is why an HTML validator will not accept a document with custom tags, or with standard tags in the wrong places (like an "img" tag in the header).

2. Will an HTML document with custom tags be parsed and rendered in a standard, clearly-defined way across browsers? Here, perhaps surprisingly, the answer is "yes." Even though the document would not technically be considered valid HTML5, the HTML5 spec does specify exactly what browsers are supposed to do when they see a custom tag: in short, the custom tag acts kind of like a - it means nothing and does nothing by default, but it can be styled by HTML and accessed by javascript.