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)

To summarize, XML is:

• simple - the complete spec is less than 40 pages long. XML is carefully designed to be easier to write software for than either HTML or full SGML.
• an open standard - XML is SGML. This means that you may not need to write any software; there are many mature, commercial tools on the market to help you author, manage, and deliver it.
• based on experience - XML was designed by people with many years of experience, including members of the governing bodies for both SGML and HTML, and people who have built successful businesses on both languages.
• extensible - you can invent and use your own tags and, if you choose, share them with others.
• efficient - XML has built-in facilities for re-using document fragments, so you only have to transmit them once.
• neutral - Nobody owns XML now, and nobody can ever own it, because SGML is a formally defined international standard.
• ready to use - Web browsers can read XML today, just like HTML (the document you are reading now is XML). You can use hyperlinks and images and multimedia, exactly as you do in HTML.
• international - XML has built-in support for texts in all the world's alphabets, including a method to signal what language and encoding you're using.
• manageable - XML includes methods for declaring and enforcing your document structures, just as with a database, so you can be sure that XML documents are created correctly.

XML is not:

• a replacement for SGML. To make XML simple, many features of SGML were left out. People who are already using SGML may choose to use XML for network delivery, but there is no real need to convert existing SGML documents. XML is designed as an easy on-ramp to SGML for people who are not using it yet.
• a replacement for HTML. HTML is an excellent tool for displaying hypermedia documents across a network. If you have XML or SGML, it is easy to generate HTML on the fly. XML is designed for electronic information providers who want to do things that HTML isn't designed for.