Doctype
A Document Type Definition [a.k.a DOCTYPE or the even shorter abbreviation DTD] is used to tell the browser what specification the document adheres too and whether to render in a standards-compliant ["strict"] or backwards-compatible, buggy mode ["quirks"]. It must appear as the first tag in the document, even before the HTML tag.
Note: !DOCTYPE must appear in captials, which may be counter-intuitive to those used to working in XHTML. Remember, it appears before the HTML document starts...
There are many DTDs out there; listed below are the ones most used for web pages.
HTML 5
<!DOCTYPE html>
XHTML 1.1 DTD
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
XHTML 1.0 Strict, Transitional, Frameset
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
HTML 4.01 Strict, Transitional, Frameset
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
Further reading and sources
- W3C's doctypes list
- W3C doctypes tips article
- Zeldman's Fix Your Site With the Right DOCTYPE! A List Apart article
