Valid against a DTD or XML Schema:

• No fabricated elements (e.g. "center" ).

• Containment is correct.

• Text is in the right place.

XHTML constructs are used in a "semantically correct" way.

CSS is used for styling.

There is no "center" tag.

Read the specification if you aren't sure.

See: http://www.w3.org/TR/xhtml11/

Don't use the break line tag: <br/>

You can do the same thing with CSS much more elegantly (and with more features).

This is not a good use of XHTML:

Here is my paragraph<p/>
Here is another one<br/>

And this is better:

<p>Here is my paragraph</p>

Don't use some jumble of elements just to get it to look right.

Use CSS to get it to look right.

For example:

<p class="author-name">Alex Milowski</p>

and the CSS:

p.author-name {
   font-size: 20pt;
   font-weight: bold;
   text-align: center;
}

There really isn't an official XML Schema for XHTML yet.

So you need to use a DTD.

If you want XHTML, put this in your stylesheet:

<xsl:output 
   method="xml" 
   doctype-public="-//W3C//DTD XHTML 1.1//EN"
   doctype-system="http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"/>

If you want HTML, put this in your stylesheet:

<xsl:output 
   method="html" 
   doctype-public="-//W3C//DTD XHTML 1.1//EN"
   doctype-system="http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"/>

The W3C can check your output for you.

See: http://validator.w3.org/

You upload you file.

Or point it at your website.

It tells you what is wrong.

The input: dream.xml

The transform: dream2xhtml.xsl

The output: dream.xhtml