I've started a Drupal Ladder style guide page at http://drupalladder.org/content/drupal-ladder-style-guide. It is linked from the Getting Involved page now. Basically we should follow the Drupal.org style guide, though I highlighted a few of those that we need to pay attention to in particular, and added a number of items that are particular to the Drupal Ladder site.
I've started cleaning up the first 5 lessons on the ladder for style, though they still need a little more work. I've also tried to take care of the comments we had by either incorporating the comments into the lesson, or adding link to external resources in the "Resources" section that address the questions.
Please feel free to help clean up the style on existing lesson, and make notes about things that should be added or changed in the Drupal Ladder style guide.
(Note, for now we are using strong as "headings" because the styling on the site is funky and readability is more important to me right now. I've created an issue at http://drupal.org/node/1772478 to make the section headers use real H2 tags, so we can use H3 tags within a section, and hopefully have it look appropriate in addition to being more semantically correct.)