More semantic tags in HTML?

Question

HTML5 introduced a variety of new semantic tags (such as <header>, <footer> and <article>) to replace divs and similar generic tags where appropriate. This seems like a good thing.

I've been experimenting with Polymer, and the Web Components spec seems like the future, albeit a little ahead of its time. In particular, its custom tag names and ability to separate data from layout makes pages of HTML extremely semantic and readable.

The components have templates that form the shadow DOM, and the templates are populated using content tags that select data elements based on their tag names. Putting HTML5's semantic tags and <content> tags together seems like a no-brainer, distributing uniquely-named elements rather than trying to insert three <div>s in three different places and relying on declaration order.

In particular, it would be great if we could use custom semantic tags for data, and use those semantic names for selection. For example, a name card component could have <content> tags that select data elements such as <name> and <description> rather than using attributes.

However, <name> and <description> are not valid tags in HTML5. Browsers treat all unrecognised tags as <span> elements, so there would be no implementation problems with allowing custom tags in the DOM. They would carry additional semantic information for readers of the code, and potentially search engine crawlers as well. Code might be less consistent, but arguably 'better'.

If this is too vague a question, I'll make it explicit: is there a definitive reason why tags such as <name> are not valid HTML5?

EDIT: I just tested the <content> selection, and verified that you can in fact use selectors other than just tag name in the select attribute. I hadn't seen this in any tutorials, but it mostly satisfies my practical desire for semantic tags. For my example, you could instead use <content select="p.name"> and <content select="p.description">.

I'm still interested from a theoretical perspective, however.

Answer 1

The HTML5 spec gives a reason, in section 3.2.1 Semantics:

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