DOCtypes (DTD)

Why Should I use a Doctype?

Using an incomplete or outdated DOCTYPE—or no DOCTYPE at all—throws some browsers into “Quirks” mode, where the browser assumes you’ve written old-fashioned, invalid markup and code just like in the late 1990s.

In this setting, the browser will attempt to parse your page in backward–compatible fashion, rendering your CSS as it might have looked in IE4, and reverting to a proprietary, browser–specific DOM. (IE reverts to the IE DOM; Mozilla and Netscape 6 revert to who knows what.)

Clearly, this is not what you want. But it is often what you’ll get, due to the incorrect or incomplete DOCTYPE information.

Flavors of Doctype

There are many doctypes available. For starters, we will use this:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

But you should aim in using:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

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

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

Notice that this DOCTYPE includes a complete URI at the end of the tag. Since the tag provides a valid location on the web, the browser knows where to find it, and will render your document in standards–compliant mode.

Here are the doctypes that usually work and is well recommended :

HTML 4.01 Strict, Transitional, Frameset

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"

"http://www.w3.org/TR/html4/strict.dtd">

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"

"http://www.w3.org/TR/html4/loose.dtd">

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"

"http://www.w3.org/TR/html4/frameset.dtd">

XHTML 1.0 Strict, Transitional, Frameset

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">

XHTML 1.1 DTD

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

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

Declare an XML namespace.

Note this line is your new html tag. Here's an example:

When a browser is handling an XHTML page and wants to know what's in the DTD, which lists and defines all the valid XHTML tags, here's where it can find itburied away on the servers of the WC3.

In short, the DOCTYPE and namespace declarations ensure that the browser interprets your XHTML code as you intended.

Declare your content type.

The content type declaration goes in the head of your document, along with any other meta tags you may add. The most common is

This simply states what character coding was used for the document. ISO-8859-1 is the Latin character set, used by all standard flavors of English, so if you are coding for an audience who uses the alphabet instead of, for example, Chinese or Farsi characters, this is the one you need. If your next site is going to be in Cyrillic or Hebrew, you can find the appropriate content types on Microsoft's site (http://msdn.microsoft.com/workshop/author/dhtml/reference/charsets/charset4.asp).

Now Lets Volt in, No I meant lets put it together

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<head>

This is how your first lines of XHTML code should look like if you want to be in the standards mode.

Writing XHTML

Because CSS is a mechanism for styling XHTML, you can't start using CSS until you have a solid grounding in XHTML. And what, exactly, is XHTML?

XHTML is based on the free-form structure of XML, where tags can be named to actually describe the content they contain; for example, <address>Manila</address>. This very powerful capability of XML means that when you develop your set of custom tags for your XML content, you also must create a second document, known as a DTD (document type definition) or a similarly formatted XML schema, to explain to the device that is interpreting the XML for how to handle those tags.

Correctly written XHTML markup gives you the best chance that your pages will display correctly in a broad variety of devices for years to come. The clean, easy-to-write, and flexible nature of XHTML produces code that loads fast, is easy to understand when editing, and prepares your content for use in a variety of applications.

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