Thanks! I didn't know that. I got rid of the extra tags in the external HTML files, but the problem is still there.
I figured it out. It seems like one position:absolute was fighting with another position:absolute, I guess. I'm still not sure why it worked, but here's what I did (in main.css). I got rid of the positioning in here:
border:1px solid purple;
margin:0 220px 0px 0; /* alternative: set this to 0 and make onecol, twocol 42% */
IIRC, I added position there in the first place because the sidebar got cut off if I resized the browser window. Now it seems to work fine. Mysterious, but that's how a lot of this stuff is to me.