HTML5: Why not use my own arbitrary tag names?

Question

As far as I know, it is perfectly legal to make up tag names in HTML5, and they work like normal with CSS styling and nesting and all.

Of course my arbitrary tag nam

Answer 1

UPDATE

The original answer is quite old and was created before web components existed (although they had been discussed since 2011). The rules on custom elements have changed quite a bit. The W3C now has a whole standard for custom elements

Custom elements provide a way for authors to build their own fully-featured DOM elements. Although authors could always use non-standard elements in their documents, with application-specific behaviour added after the fact by scripting or similar, such elements have historically been non-conforming and not very functional. By defining a custom element, authors can inform the parser how to properly construct an element and how elements of that class should react to changes.

In other words, the W3C is now fine with you using custom elements (or arbitrary tag names as the question calls them). However, there are quite a few rules surrounding them and their use so you should refer to the linked documentation.

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Original answer

1. Older browsers choke on tags they don't recognize. There is nothing that guarantees that your custom tags will work as you expect in any browser
2. There is no semantic meaning applied to your tags since this meaning only exists in the agreed upon tags in the spec. Whether or not this affects SEO is unknown, but anything that parses the HTML may have trouble if it runs into a tag it doesn't recognize including screen readers or web scrapers
3. You may not get the help you need from the browser if your tags are improperly nested
4. In the same vein, some browsers may screw up rendering and be different from one another. For example, <p><ul></ul></p> is parsed in Chrome as <p></p><ul></ul>. There is no guarantee how your tags will be treated since they have no agreed upon content model
5. The W3C doesn't want you to do it.

   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.

If you really need to use custom tags for this purpose, you can use a template language of some sort such as XSLT and create a stylesheet that transforms it into valid HTML