I've had my WordPress blog at http://www.nostalgia101.com/ installed for a while now, though I've never had it completely validated in xhtml since I added that different doctype, which is <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> and I read about this in another older post. Anyway, changing the doctype to this did make things easier for me when I designed the layout of my blog. However, I've decided that I'd like to try getting it to validate in xhtml again, and I've spent the past week going through not only my blog, but all my other pages of my site to fix my coding. I've gotten help from this tutorial: http://www.w3schools.com/xhtml/xhtml_intro.asp and I thought I had everything correct, including my blog, so when I tried to change the doctype from HTML 4.01 Transitional back to the way it was before, XHTML 1.0 Transitional, the layout for my blog and for my other pages gets messed up. The only wordpress pages I've changed since I installed it have been the index.php and wp-comments.php files.
I'm assuming that it has something to do with my stylesheet at http://www.nostalgia101.com/stylesheet.css, but I'm not sure what my mistake(s) could be. I've tried looking through the w3schools.com website for some tutorial on CSS to see if I have been doing it right or if I made some huge mistakes, but so far the stuff I've found doesn't seem to make sense to me.