Web Design in a Nutshell, 3rd Edition

Extending HTML 4 and XHTML

By following many of the techniques recommended in this book, you may already be extending HTML for your own purposes without even knowing it. Chapter 10 discusses how to provide more semantic descriptions of your content using the class and id attributes. Every meaningful class name and ID extends the semantics expressed by your HTML documents.

HTML 4 has three built-in extension mechanisms. In addition to using semantic class and id attributes, web authors can create their own link relationships with the rel and rev attributes, and property names and values for use with the meta tag. Two of these mechanisms, class/id and rel/rev, are being leveraged into microformats. The third, meta, shows less promise, as is discussed in the sidebar "meta Names and Values..”

meta Names and Values

The HTML 4 specification discusses how to extend the meta-information provided in a document by defining and using new meta property names and values. There have been various efforts to standardize such efforts (e.g., Dublin Core). Because the invisible meta-information in the head of a document is often disconnected from and out of sync with the visible content of a document, however, these methods have proved to be problematic in practice, especially as documents age or are maintained by multiple authors. Even meta keywords, once used by all search engines, are ignored by Google and others. In fact, most authors no longer waste time or bandwidth with meta keywords in web documents. ...