If the declaration is not made, the browser will do its best to render the page accordingly. If you’ve been designing web pages for a while now, you are aware of the difficulty in developing a page that looks the same in all browsers. By setting the DOCTYPE, you can ensure that your web pages will be rendered in a similar format because this instructs the browser exactly how to handle the elements it finds in the document. The declaration refers to a Document Type Definition (DTD). The DTD specifies the rules for the markuplanguage. Again, this is so that the browsers render the content correctly.
Parts of a DOCTYPE
A DOCTYPE is made up of the following parts:
!DOCTYPE: It indicates to the user-agent that the enclosed information will define the type of document of the page.HTML: This tells the browser what element to expect as the top-level element. For HTML and XHTML documents this element would be.PUBLIC: The most common DOCTYPES you will use will be publicly available. You can also specify a local DTD with the SYSTEM keyword."-//W3C//DTD HTML 4.01 Transitional//EN": This is the Formal Public Identifier (FPI). This entire string is what identifies the DOCTYPE.
More on FPI
The FPI is made up of these parts:
-/+: A plus-sign indicates that the organization is registered with the ISO. A minus-sign indicates that it is not registered.W3C: This is the group that owns and maintains the DOCTYPE being used.DTD: This defines the type of DOCTYPE used.HTML 4.01 Transitional: This is the label that tells you what DTD is being used in readable form.EN: This is the language that the DTD is written in. It is not the language of the content of the page.http://www.w3.org/TR/html4/loose.dtd: This is an optional URL indicating where the DTD for this DOCTYPE can be found.
Sample DOCTYPEs:
HTML 2.0
HTML 3.0
HTML 3.2
HTML 4.01
Strict DTD – contains all HTML elements and attributes, but does not include presentational or deprecated elements. Framesets are not allowed. Transitional – contains all HTML elements and attributes, including presentational and deprecated elements. Framesets are not allowed. Frameset DTD – equal to HTML 4.01 Transitional, but does allow the use of frameset content.
XHTML 1.0
Strict DTD – contains all HTML elements and attributes, but does not include presentational or deprecated elements. Framesets are not allowed. Transitional – contains all HTML elements and attributes, including presentational and deprecated elements. Framesets are not allowed. Frameset DTD – equal to XHTML 1.0 Transitional, but does allow the use of frameset content.
XHTML 1.1
Equal to XHTML 1.0. Strict, but allows you to add modules.
XHTML 2.0
HTML 5
HTML5 is not based on SGML and therefore does not require a reference to a DTD. Reminder: Always add the declaration to your HTML documents, so that the browser knows what type of document to expect.
