XML parser and linter that supports XPath, a syntax for navigating XML trees.
xmllint --xpath "//{{foo}}" {{source_file.xml}}
xmllint --xpath "string(//{{foo}})" {{source_file.xml}}
xmllint --html --xpath "string(//a[2]/@href)" webpage.xhtml
xmllint --format {{source_file.xml}}
xmllint --valid {{source_file.xml}}
xmllint --dtdvalid {{URL}} {{source_file.xml}}