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Doctypes

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.

Adding the Doctype

Section titled “Adding the Doctype”

The <!DOCTYPE> declaration should always be included at the top of the HTML document, before the <html> tag.

See HTML 5 Doctype for details on the HTML 5 Doctype.

<!DOCTYPE html>

See HTML 4.01 Doctypes for details on how these types differ from each other.

Strict

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

Transitional

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

Frameset

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">

HTML 5 Doctype

Section titled “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

Section titled “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 `"

    therefore the following DOCTYPEs are also valid:

    <!doctype html>
    <!dOCtyPe html>
    <!DocTYpe html>

    This SO article discusses the topic extensively: Uppercase or lowercase doctype?

    HTML 4.01 Doctypes

    Section titled “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

    Section titled “HTML 4.01 Strict”
    <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://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

    Section titled “HTML 4.01 Transitional”
    <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://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

    Section titled “HTML 4.01 Frameset”
    <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">

    Includes all HTML elements and attributes, presentational and deprecated elements. Framesets are allowed.

    Old Doctypes

    Section titled “Old Doctypes”

    HTML 3.2

    Section titled “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

    Section titled “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.

    Syntax

    Section titled “Syntax”

    Remarks

    Section titled “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.