The DOCTYPE declaration is used to inform the visitor’s web browser that the document being rendered is an HTML document. Every HTML document should be with a DOCTYPE declaration to be compliant with HTML standards. Without the DOCTYPE, we are forcing the browsers to render in Quirks Mode. DOCTYPE is partially used by the browsers to determine dialect and parsing. This is why HTML5 has reduced the DOCTYPE to simply.
A document type declaration, or DOCTYPE, is an instruction that associates a particular SGML or XML document (for example, a webpage) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML1.0 – HTML 4.0). In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax.
HTML 5
<!DOCTYPE html>
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">
There were some other doctype declarations in previous HTML versions, for more details visit W3C
Doctype is very important for an HTML document and it is best practice to always use doctype at the beginning of your HTML document.
