Safari 5.1.8 in Snow Leopard 10.6.8 will not display HTML pages correctly
I have solved my own problem.

Enough snooping around for .plist files got the job done. I identified all the .plist files with "Safari" and "com.apple.Safari" in the name in ~/Library/ and /Library/, and moved them to the Trash. Then I found a couple of invisible files in /Library/ with "Safari" in them and likewise moved them to the Trash. Then I rebooted. Safari now works correctly.

I wish I had thought of that first. I have wasted hours on this problem.