DTP's Doctype-Declaration (DTD)
Each document has to begin with a doctype-declaration (DTD). It specifies which HTML-Standard will be used. The DTDs, Document Type Definitions, are a consistent part of HTML and XHTML. The doctype is situated at the top of the HTML-file just above the <html> tag.
HTML 5
HTML5 DOCTYPE is not case-sensitive. The following notations are all correct.
<!DOCTYPE html> <!doctype html> <!Doctype HTML>
HTML5 is not based on SGML and doesn't require any DTD specifications.
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">
Like HTML 4.01, XHTML 1.0 also knows three diffrent types of DTDs. But unlike HTML 4.01 it is absolutely necessary, that each document is introduced by a doctype declaration.
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">
XHTML 1.1 DTD
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">