Hi Harvey,
The answer to your question is in first line of the source of the page. The DOCTYPE shows:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
but I fear you are out to embarrass the good folks at Nabble as a validation check at the W3C site shows that with just your initial post on this page there were 23 errors and 21 warnings about compliance with the standard that was claimed to be used. I conclude that Nabble have not been too worried about maintaining sites that follow HTML guidelines closely and a change to HTML5 will be low on the priority list.
(Failure to deliver valid code will be part of the reason why there are sometimes complaints that something doesn't display well in browser X or Y.)
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.