Workshop - Doctypes

Part 2 - (X)HTML

Doctypes

What are they and why are they important?

All HTML and XHTML documents must have a doctype declaration to be valid.
The doctype states the version of HTML or XHMTL is being used in the document.
Doctypes are used by the validator when validating, and by web browsers to determine which rendering mode to use.
Doctypes affect the way devices render web pages.
If a correct and full doctype is present in a document, many web browsers will switch to standards mode, which means that they will follow the CSS specification closer.
Documents with correct and full doctypes will also render quicker because the browser doesn't have to interpret and try to compensate for invalid 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">

XHTML 1.1

	<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"

	"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

Standards compliant (or strict) and quirks modes

Most browsers run in two modes: quirks mode for the old rules and strict mode for the standard. Mozilla, Safari, Opera, Mac/Explorer, Windows/Explorer 6 implemented these two modes. Windows/Explorer 5 and older browsers like Netscape 4, are permanently locked in quirks mode.

There are some very important reasons to use strict, or standards compliant mode. For example, Windows/Explorer 6 will use the correct box model when in strict mode, but the incorrect box model when in quirks mode. Also, many modern browsers will not allow fonts to be inherited when in quirks mode.

Is the XML prolog necessary?

Using the XML prolog in XHTML documents is a practice that is recommended but not required by the W3C. So, it's up to developers whether to use it or to leave it out. If the prolog is included, Windows/IE6 will revert to quirks mode regardless of the doctype used. The prolog looks like this:

<?xml version="1.0" encoding="utf-8"?>