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The XML FAQ — Frequently-Asked Questions about the Extensible Markup Language
Section 3: Authors
Q 3.1: Do I have to know HTML or SGML before I learn XML?
No, but it's useful.
You don't need any foreknowledge, but it's useful because a lot of XML concepts, terminology, and practice derive from two decades' experience of SGML (and the systems which came before it).
Be aware that ‘knowing HTML’ is not the same as ‘understanding SGML’. Although HTML was written as an SGML application, browsers ignore most of it (which is why so many useful things don't work), so just because something is done a certain way in HTML browsers does not mean it's correct SGML or XML.
For most practical purposes you don't need any knowledge of SGML or HTML: only if you intend delving into the internals of document type design.