I use the frame-based solution for doc lookup (index.html by me), here
<http://www.software-lab.de/doc/>, quite a lot, but for some time now I have
noticed that some modern browsers (eg. Chrome, Safari) are complaining about
this code, but when I checked the URL with <http://validator.w3.org>, I got
quite a few red errors. I then found that almost all of these validator errors
could be made to go away if I replaced the first two lines in index.html with
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
early to say.