Namespaces

• XPath /root/child fails if document has default namespace—use //*[local-name()='child'] or register prefix
• Default namespace (xmlns="...") applies to elements, not attributes—attributes need explicit prefix
• Namespace prefix is arbitrary—<foo:element> and <bar:element> are identical if both prefixes map to same URI
• Child elements don't inherit parent's prefixed namespace—each must declare or use prefix explicitly

Encoding

• <?xml version="1.0" encoding="UTF-8"?> must match actual file encoding—mismatch corrupts non-ASCII
• Encoding declaration must be first thing in file—no whitespace or BOM before it (except UTF-8 BOM allowed)
• Default encoding is UTF-8 if declaration omitted—but explicit is safer across parsers

Escaping & CDATA

• Five entities always escape in text: & < > " '
• CDATA sections <![CDATA[...]]> for blocks with many special chars—but ]]> inside CDATA breaks it
• Attribute values: use " if delimited by ", or ' if delimited by '
• Numeric entities < and < work everywhere—useful for edge cases

Whitespace

• Whitespace between elements is preserved by default—pretty-printing adds nodes that may break processing
• xml:space="preserve" attribute signals whitespace significance—but not all parsers respect it
• Normalize-space in XPath: normalize-space(text()) trims and collapses internal whitespace

XPath Pitfalls

• //element is expensive—traverses entire document; use specific paths when structure is known
• Position is 1-indexed: [1] is first, not [0]
• text() returns direct text children only—use string() or . for concatenated descendant text
• Boolean in predicates: [@attr] tests existence, [@attr=''] tests empty value—different results

Structure

• Self-closing <tag/> and empty <tag></tag> are semantically identical—but some legacy systems choke on self-closing
• Comments cannot contain --—will break parser even inside string content
• Processing instructions <?target data?> cannot have ?> in data
• Root element required—document with only comments/PIs and no element is invalid

Validation

• Well-formed ≠ valid—parser may accept structure but fail against schema
• DTD validates but can't express complex constraints—prefer XSD or RelaxNG for new projects
• XSD namespace xmlns:xs="http://www.w3.org/2001/XMLSchema" commonly confused with instance namespace