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What the hell is <!DOCTYPE html>?
The page says that older HTML documents used a doctype declaration with a reference to a DTD, which described the document structure and the valid elements it could contain. It then explains that HTML5 no longer needs that SGML-based reference, but the doctype still matters because it tells browsers to run the page in standard mode.
Reading notes#
- A DTD describes the structure of an XML document and the legal elements it can contain.
- Before HTML5, HTML documents used a doctype declaration with a DTD reference on the first line.
- That declaration provided the list of valid elements and identified whether the DTD was strict, transitional, or frameset.
- HTML5 no longer needs a DTD reference because it is not SGML based.
- In HTML5,
<!DOCTYPE html>tells the browser to use standard mode. - Without a valid doctype on the first line, the browser may switch to quirks mode.
- The page links quirks mode to older browsers and to legacy code that broke when standards-based behavior became common.