Doctypes
Introduction#
Doctypes - short for ‘document type’ - help browsers to understand the version of HTML the document is written in for better interpretability. Doctype declarations are not HTML tags and belong at the very top of a document. This topic explains the structure and declaration of various doctypes in HTML.
Syntax#
- <!DOCTYPE [version-specific string]>
Remarks#
The <!DOCTYPE> declaration is not an HTML tag. It is used for specifying which version of HTML the document is using. This is referred to as the document type declaration (DTD).
The <!DOCTYPE> declaration is NOT case sensitive. To check if the HTML of your Web pages is valid, go to W3C’s validation service.
-
Some old versions of IE don’t support some HTML tags unless a proper doctype is available.
-
It’s vital that a doctype is declared as to make sure the browser doesn’t use quirks mode. More info on MDN.
Adding the Doctype
The <!DOCTYPE> declaration should always be included at the top of the HTML document, before the <html> tag.
See https://stackoverflow.com/documentation/html/806/doctypes/16623/html-5-doctype for details on the HTML 5 Doctype.
<!DOCTYPE html>See https://stackoverflow.com/documentation/html/806/doctypes/3148/html-4-01-doctypes for details on how these types differ from each other.
Strict
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "https://www.w3.org/TR/html4/strict.dtd">Transitional
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "https://www.w3.org/TR/html4/loose.dtd">Frameset
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "https://www.w3.org/TR/html4/frameset.dtd">HTML 4.01 Doctypes
The HTML 4.01 specification provides several different types of doctypes that allow different types of elements to be specified within the document.
HTML 4.01 Strict
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "https://www.w3.org/TR/html4/strict.dtd">Includes all HTML elements and attributes, but does not include presentational or deprecated elements and framesets are not allowed.
HTML 4.01 Transitional
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "https://www.w3.org/TR/html4/loose.dtd">Includes all HTML elements and attributes and presentational and deprecated elements, but framesets are not allowed.
HTML 4.01 Frameset
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "https://www.w3.org/TR/html4/frameset.dtd">Includes all HTML elements and attributes, presentational and deprecated elements. Framesets are allowed.
HTML 5 Doctype
HTML5 is not based on SGML, and therefore does not require a reference to a DTD.
HTML 5 Doctype declaration:
<!DOCTYPE html>Case Insensitivity
Per the W3.org HTML 5 DOCTYPE Spec:
A DOCTYPE must consist of the following components, in this order:
- A string that is an ASCII case-insensitive match for the string
"<!DOCTYPE".
therefore the following DOCTYPEs are also valid:
<!doctype html>
<!dOCtyPe html>
<!DocTYpe html>This SO article discusses the topic extensively: https://stackoverflow.com/questions/7020961/uppercase-or-lowercase-doctype
Old Doctypes
HTML 3.2
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">HTML 3.2 is well supported by most browsers in use. However, HTML 3.2 has limited support for style sheets and no support for HTML 4 features such as frames and internationalization.
HTML 2.0
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">HTML 2.0 is widely supported by browsers but lacks support for tables, frames, and internationalization, as well as many commonly used presentation elements and attributes.