The DOCTYPE element is an identifier found at the very beginning of an HTML document, even before the element.
DocType declares to the browser what version of (X)HTML (eg. 3.4 or 4.01, 5.0 etc.) is used in the document. DocType is not an HTML Tag.
Syntax of DocType and Explanation:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
!DOCTYPE: The identifier. It indicates to the user-agent that the enclosed information will define the type of document of the page.
HTML: The Top Element. This tells the browser what element to expect as the top-level element. For HTML and XHTML documents this element would be <html>
PUBLIC: The Availability. The most common DOCTYPES you will use will be publicly available. But you can also specify a local DTD with the "SYSTEM" key word.
"-//W3C//DTD HTML 4.01 Transitional//EN": The Formal Public Identifier (FPI).
The FPI is made up of these parts:
Registration. If there is a plus-sign (+) here, that means that the organization is registered with the ISO. A minus-sign (-) indicates that it is not registered.
W3C: The Organization. This is the group that owns and maintains the DOCTYPE being used.
DTD: The Type. This defines the type of DOCTYPE used. A Document Type Definition (DTD) defines the legal building blocks of an XML/HTML document. It defines the document structure with a list of legal elements and attributes.
HTML 4.01 Transitional: The Human-Readable Label. This is the label that tells you what DTD is being used. It is written so that humans, rather than computers, can understand it.
EN: The Language. This is the language that the DTD is written in. It is not the language of the content of the page.
The rest of the DOCTYPE identifier is optional: "http://www.w3.org/TR/html4/loose.dtd": It is used to indicate where the DTD for this DOCTYPE can be found
DocType is also used to describe the particular flavor of HTML you're using.
For example: There are three variants on HTML 4.01:
1. HTML 4.01 Strict, which describes the structural portions of HTML 4.01 and does not provide any presentational markup or frame-related markup. HTML 4.0 Strict says you to use an Cascading Style Sheet for presentational markup (<b>, <font> etc.) and not to include these elements in HTML page. This DocType claims that the document is a strict document; that is, it is authored according to a strict adherence to the W3C specification, and uses no presentational markup (<b>, <font> etc.). Upon seeing this, IE5/Mac will kick its rendering engine into STANDARD MODE, so that your document will be displayed according to the W3C standards.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
2. HTML 4.01 Transitional, which includes the presentational markup such as and , but does not include frame-related markup. Transitional DTD allows some older PUBLIC and attributes that have been deprecated. This declaration calls for transitional (or loose) adherence to the standards. In this case, IE5/Mac will display the document according to bugwards compatibility rules (Quirk Mode). This will bring the rendering more in line with Internet Explorer for Mac's historical behavior. Upon seeing this, IE5/Mac will kick its rendering engine into QUIRK MODE.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
You'll notice that neither example included the URL of a DTD. This was on purpose. If you include the URL of a DTD on any DOCTYPE, even the loose and frameset kinds, then IE5/Mac will go into STRICT MODE. This may lead to unexpected results, as the rendering engine will no longer attempt to emulate historical browser flaws.
Note: There are two types of webpage rendering mode of a brower: Standard Mode and Quirk Mode.
3. HTML 4.01 Frameset, which is the same as HTML 4.01 Transitional except that it adds a description of frame-related markup. If frames are used, the Frameset DTD must be used.
HTML5 DTD-less DOCTYPE
HTML5 uses a DOCTYPE declaration which is very short, due to its lack of references to a Document Type Definition in the form of a URL and/or FPI. All it contains is the tag name of the root element of the document, HTML.
What if DocType is absent in HTML document?
1. An HTML document which lacks a DOCTYPE, will be rendered in bugwards compatibility mode (Quirk Mode), since it is assumed to be an older document which was written before DOCTYPE became widely used.
2.You will not be able to use a HTML (HyperText Markup Language) Validator to check the page coding. HTML validator requires the DOCTYPE declaration.
3.The stylesheet may not be implemented as planned.