The Decoration XSD is located here.
Your favorite IDE probably supports XSD schema's for site.xml files. You need to specify the following:
<project xmlns="http://maven.apache.org/DECORATION/1.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/DECORATION/1.0.0 http://maven.apache.org/xsd/decoration-1.0.0.xsd"> ... </project>
