Since XML and HTML are derived from SGML they are similar, but have the following differences:
1. XML is case-sensitive
2. XML must have quotes (single or double) around attributes
3. Most interpreters of HTML are very forgiving about missing end tags - XML parses are not.
4. Comments start with <-- and end with -->. Inside a comment, "--" may not appear. Although this is fine in html, it confuses xml parsers.