HTML and CSS Reference
1.5.3. Avoiding Extensions
In general, we urge you to resist using extensions unless you have a
compelling and overriding reason to do so. By using them, particularly in
key portions of your documents, you run the risk of losing a substantial
portion of your potential readership. To be fair, most browsers eschew
extensions, so the point is moot now.
We admit that it is disingenuous of us to decry the use of extensions
while presenting complete descriptions of their use. In keeping with the
general philosophy of the Internet, we'll err on the side of handing out
rope and guns to all interested parties while hoping you have enough
smarts to keep from hanging yourself or shooting yourself in the foot.
Our advice still holds, though: use an extension only where it is ne-
cessary or very advantageous, and do so with the understanding that
you are disenfranchising a portion of your audience. To that end, you
might even consider providing separate, standards-based versions of
your documents to accommodate users of other browsers.
1.5.4. Extensions Through Modules
XHTML version 1.1 provides a mechanism for extending the language in
a standard way: XML modules. In fact, XHTML 1.1 is composed of mod-
XHTML modules divide the HTML language into discrete document types,
each defining features and functions that are parts of the language.
There are separate modules for XHTML forms, text, scripting, tables,
and so onall the nondeprecated elements of XHTML 1.0.
The advantage of modules is extensibility. In addition to using the
markup features from the XHTML modules normally included in the
standard, the new language lets you easily blend other XML modules in-
to your documents, extending their features and capabilities in a stand-
ard way. For instance, the W3C has defined a MathML module that
provides explicit markup elements for mathematical equations that you
could use in your next XHTML-based math thesis.