Unfortunately, there are things that HTML just can't do for you.
Fortunately, HTML is growing quickly to meet these needs.
Unfortunately, no matter how many new tags are added, there will never be enough for all the good ideas people keep having.
Fortunately, HTML is a form of SGML (Standard Generalized Markup Language), an ISO standard that allows you to invent the tags you need, and declare them so others can use them.
Unfortunately, the SGML standard is large, takes time to learn, and doesn't have a "starter kit".
Fortunately, XML is here.

Extensible Markup Language (XML)

Resources: The FAQ
Peter Flynn's FAQ, all the way from Ireland
The spec
XML resources from Robin Cover's SGML home page
My XML Processor, "Lark"

To summarize, XML is:

XML is not: