XML.com FAQs > C. Authors of SGML (including writers of HTML: Web page owners)
Question:  C.9 I'm used to authoring and serving HTML. Can I learn XML easily?

Yes, very easily, but at the moment there is still a need for tutorials, simpler tools, and more examples of XML documents. Well-formed XML documents may look similar to HTML except for some small but very important points of syntax.

The big practical difference is that XML has to stick to the rules. HTML browsers let you serve them broken or corrupt HTML because they don't do a formal parse but elide all the broken bits instead. With XML your files have to be correct or they simply won't work at all. One outstanding problem is that some browsers claiming XML conformance are also broken. Try yours on the test file at http://www.ucc.ie/test.xml.

This FAQ is from The XML FAQ, maintained by Peter Flynn