@Richard, your not completely right (<< edit, error in my initial post)
An HTML skin and an XHTML skin can be equally sloppy.
The thing is that most people who create XHTML skins, care about validation, so they make sure there skins are VALID XHTML.
XHTML is not difficult it is actually easier since Internet Explorer behaves better with that doctype an you needs less CSS hacks.
So it takes a bit more work to create an XHTML complaint skin (and everybody should IMO), but even if the skin is not 100% compliant the skin will still work, just like a "sloppy" HTML skin does.
XHTML valid skins are not difficult IMO, here are the most important things to consider:
(from this blog)
-All tags should be closed
Wrong: <br>
<hr>
<img src="test.gif" alt="test">
Correct: <br />
<hr />
<img src="test.gif" alt="test" />
-All attributes should be in lowercase
Wrong: <div Class="test">
<div onClick="myfunction">
Correct: <div class="test">
<div onclick="myfunction">
-All Attribute values should be quoted:
Wrong: <td colspan=2>
Correct: <td colspan="2">
-All image tags should have alt attribute defined.
Wrong: <img src="test.gif" />
Correct: <img src="test.gif" alt="test image" />
-Some HTML elements have been deprecated
Wrong: <center></center>
Correct: <div class="DNNAligncenter"></div>
-You cannot use Block elements inside inline elements:
Wrong: <span><div>test</div></span>
Correct: <div><div>test</div></div>
This is often caused by the use of the label control, use the literal control instead (with a div). If you have to add a block element inside an inline element, use an inline element and set its CSS to display:block, the result is the same and it will validate. Please note that a <p> element can not contain any block element either so use divs.
-Script tags must have a type="text/Javascript"
Wrong: <script src="myscript.js" language="javascript"></script>
Correct: <script src="myscript.js" type="text/javascript"></script>
- Links to external CSS stylesheet inside the body of the page are not compliant
A Link to an external stylesheet must be in the <head> of the page to be compliant
For modules, move all this CSS to module.css, if it's dynamic CSS, write it in the head of the page.
Skins should have all CSS in either in skin.css of "skinname".css