Pixel Pete saysXHTML differs from HTML in five rules: lowercase, close everything, quote attributes, nest correctly, one root.
The practical differences from HTML: tag and attribute names must be lowercase; every element must be closed (including void ones as <br />); attribute values must be quoted; elements must nest without overlap; and the document needs a single root with the XHTML namespace. Break any rule and a true XHTML parser rejects the page.
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Lowercase tags & attributes
Close every element (void ones self-close)
Quote all attribute values
Nest cleanly; one root element
Quirks Mode attacks — common mistakes
Uppercase tags (valid HTML, invalid XHTML)
Unclosed <br>, <img>, <input>
Unquoted attribute values
Boss battleTake an HTML line breaking three of these rules and fix all three.