Question

one main difference between XML and HTML

Answer 1

XML

Here tags are user define

HTML

Here tags are predefine

Answer 2

XML stands for xtensible markup language. XML has been designed to describe data and to focus on what data is. XML allows the web developer to define his own tags and document structure. XML is derived from SGML.
HTML stands for hyper text markup language. HTML has been designed to display data and focus on how data looks.The structure and markup tags used in HTML documents are predefined. HTML is an application of SGML.