XHTML Syntax Rules Explained
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The Strict Rules of XHTML
Because XHTML is XML, it enforces rules that HTML treats as optional. Breaking any of them makes the document invalid.
1. All Tags Must Close
<p>Paragraph</p>
<br /> <!-- self-closing with space-slash -->
<img src="x.jpg" alt="x" />2. Lowercase Everything
<DIV> ❌
<div> ✅3. Quote All Attributes
<input type=text> ❌
<input type="text" /> ✅4. Properly Nest Tags
<b><i>text</b></i> ❌ wrong order
<b><i>text</i></b> ✅5. Required Doctype & Root
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" ...>
<html xmlns="http://www.w3.org/1999/xhtml">FAQs
What happens if I break a rule?
A strict XHTML parser shows an error and may refuse to render. More in our XHTML section.
Are these rules in HTML5?
Optional, but following them keeps your markup clean.