The XHTML family is designed to accommodate extensions through XHTML modules. These modules will permit the combination of existing and new feature sets when developing content and when designing new user agents.
Alternate ways of accessing the Internet are constantly being introduced. The XHTML family is designed with general user agent interoperability in mind.
Through a new user agent and document profiling mechanism, servers, proxies, and user agents will be able to perform best effort content transformation. Ultimately, it will be possible to develop XHTML-conforming content that is usable by any XHTML-conforming user agent.