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HTML/CSSBeginner#html#meta
Standards mode renders pages according to W3C specifications, triggered by a valid DOCTYPE. Quirks mode emulates non-standard behavior of old browsers (like Netscape/IE), causing inconsistent box-model and CSS handling, triggered by a missing or malformed DOCTYPE.
<!-- Missing DOCTYPE triggers quirks mode --> <html><body>Content</body></html> <!-- Correct: standards mode --> <!DOCTYPE html> <html><body>Content</body></html>