There are several Document Type Declarations (DTDs) or "Doctypes" used in HTML. They were historically used to define the version of HTML being used and to help browsers render web pages correctly. However, with HTML5, the doctype declaration has been simplified to <!DOCTYPE html>, which triggers standards mode in modern browsers. This single doctype declaration is used for all HTML5 documents.
Here are some examples of older doctype declarations that were used in earlier versions of HTML:
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HTML 4.01 Strict:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> -
HTML 4.01 Transitional:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> -
HTML 4.01 Frameset:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd"> -
XHTML 1.0 Strict:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> -
XHTML 1.0 Transitional:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> -
XHTML 1.0 Frameset:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd"> -
XHTML 1.1:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
Note: HTML5 introduced a simplified, backward-compatible doctype declaration (<!DOCTYPE html>) that is used universally for HTML5 documents.
