The Developer Tutorials blog has posted a series they've worked up to show bloggers out there running WordPress how to create a custom theme from scratch:

In this multi-part series I'll detail how to create and design a Wordpress theme from nothing more than your imagination using Photoshop, CSS, XHTML and PHP.

Here's the list so far:

• Part 1 - getting the tools and an overall layout idea
• Part 2 - splitting it up to make the layout parts
• Part 3 - converting the Photoshop template to XHTML
• Part 4 - cleaning up the XHTML and doing some styling with CSS
• Part 5 - how to preload some images with Javascript to make the page load faster
• Part 6 - creating the WordPress-specific markup
• Part 7 - working up the PHP functionality
• Part 8 - integrating PHP into the template
• Part 9 - marking up the "framework" of the site (header/footer/sidebar)
• Part 10 - adding additional files like a links page and changing the posting template
• Part 11 (?) - deploying the theme out to the public view

In this new tutorial from DevShed, they demonstrate the use of a handy little bit of functionality to help keep your code (and markup) clean - Tidy.

Now that you know that the Tidy (X)HTML formatting/correcting application can be called directly from your own PHP 5 scripts, over the course of this series, which is comprised of three friendly tutorials, I'm going to walk you through using the bunch of useful functions included with this library.

This tutorial (part one) talks about the parsing of (X)HTML strings and using the tidy_clear_repair, tidy_parse_file and tidy_repair_file functions to handle the cleanup of (X)HTML strings and files automatically.