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Sams Teach Yourself XML in 21 Days

Sep 16, 2005

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Writing XHTML Documents

As an XML author, there are a few rules you need to know when it comes to writing XHTML documents. The following are the requirements a document must meet to be an XHTML document, according to the W3C:

The document element must be <html>.
The XHTML document must validate against one of the W3C XHTML DTDs.
The document element, <html>, must use the http://www.w3.org/1999/xhtml namespace, using the xmlns attribute.
The document must have a <!DOCTYPE> element, and it must appear before the document element.

Here's a list of some of the main things you, as HTML authors, need to watch out for when creating XHTML documents:

Element and attribute names have to be in lowercase.
Attribute values must be in quotes.
Non-empty elements need end tags. While you can sometimes omit end tags for non-empty elements in HTML, you can't in XHTML.
You cannot use standalone attributes (that is, attributes that are not assigned values) in XHTML. If you have to, you can assign a dummy value to an attribute (for example, noborder = "noborder").
An empty element needs to be ended with />. The HTML browsers don't have a problem with this ending (as opposed to just >).
The <a> element may not contain other <a> elements.
The <button> element may not contain the <input>, <select>, <textarea>, <label>, <button>, <form>, <fieldset>, <iframe>, or <isindex> elements.
The <form> element may not contain other <form> elements.
The <label> element may not contain other <label> elements.
The <pre> element may not contain <img>, <object>, <big>, <small>, <sub>, or <sup> elements.
You can use the id attribute, but you cannot use the name attribute. In XHTML 1.0, the name attribute of the <a>, <applet>, <form>, <frame>, <iframe>, <img>, and <map> elements has been deprecated. This can be a problem because browsers such as Netscape Navigator support name but not id (in which case the best solution is to use both attributes in the same element, even though it's not legal XHTML).
You must escape sensitive characters. For example, when an attribute value contains an ampersand (&), the ampersand should be given as the entity reference &.

Tomorrow you'll talk about a few more requirements (for example, if you use < characters in <SCRIPT> elements, you should either escape such characters as < or, if the browser can't handle that, place the script in an external file).

Dissecting the Example

Now let's start taking apart the XHTML document ch11_02.html to see what makes XHTML tick.

You start as you would in any XML document, with an XML declaration:

<?xml version="1.0" encoding="UTF-8"?>
    .
    .
    .

The next element is the <!DOCTYPE> element, to indicate which XHTML DTD you're using—in this case, XHTML 1.0 Transitional (which is the closest to the version of HTML in general use):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

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

This is just a standard <!DOCTYPE> element, and it indicates that the document element in the XHTML document is <html>. Remember that there is a different DTD for each version of XHTML, and they're all public DTDs, created by the W3C. The formal public identifier (FPI) for this DTD is "-//W3C//DTD XHTML 1.0 Transitional//EN", which is the DTD for XHTML 1.0 Transitional. You also list the URI for this DTD, for the benefit of XML processors:

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

These are the <!DOCTYPE> elements you should use in XHTML 1.0 for the Strict, Transitional, and Frameset DTDs, respectively:

<!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">

Note that if you're validating XHTML documents against these DTDs, you can download them and store them locally for faster access. For example, if you store these DTDs in a directory named storage, it might look like this:

<!DOCTYPE html
     PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
     "storage/xhtml1-strict.dtd">

<!DOCTYPE html
     PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
     "storage/xhtml1-transitional.dtd">

<!DOCTYPE html
     PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
     "storage/xhtml1-frameset.dtd">

Here's the <!DOCTYPE> element for XHTML 1.1 (there's only one XHTML 1.1 DTD, not three, as in XHTML 1.0, because XHTML uses Strict XHTML and doesn't have any Transitional forms):

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
     "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

Here's the <!DOCTYPE> element for XML Basic:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
     "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">

And here's the <!DOCTYPE> element for XHTML 2.0 (note that the XHTML 2.0 DTD hasn't been posted yet, so the W3C lists the URI as to-be-determined, "TBD"):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 2.0//EN" "TBD">

The DTDs at the URIs given by these <!DOCTYPE> elements are real DTDs and will work in XML processors. If possible, you should download them and use them locally, however. Imagine the bottleneck that would result from a million browsers all trying to download these DTDs at once.

Following the <!DOCTYPE> element is the <html> element, which is the document element for all XHTML documents. Note the lowercase here—<html>, not <HTML>. All elements in XHTML (except the <!DOCTYPE> element) are lowercase. That's the way XHTML works, and if you're used to using uppercase HTML tag names, XHTML tags will take a little adjustment. Here's what the <html> element looks like:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
    .
    .
    .

In this case, you're putting the entire document into the http://www.w3.org/1999/xhtml namespace, which is the official W3C namespace for XHTML documents. This element also has an xml:lang attribute, to set the language for the document when it's interpreted as XML, and the standard HTML attribute lang, to set the language when the document is treated as HTML.

The rest of this XHTML example is very much like its HTML counterpart, with the exceptions that all element names are in lowercase and the <BR> element has become the more proper <br/> XHTML element:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>

           <title>

               An XHTML Document

           </title>

       </head>


       <body>

           <h1>

               Long Live XHTML!

           </h1>

           This is an XHTML document.

           <br/>

           Pretty good, eh?

       </body>

   </html>

That's your first XHTML document. So how about validating it?

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