This site is built on the principle of keeping things accessible for the greatest number of people. It should be readable with all current and recent browsers, and indeed with most browsers of the last ten years. Those few people still using Internet Explorer 6 will get a somewhat less good layout and bugs in it sometimes cause problems such as disappearing text.
Most pages are valid HTML 4.01, except that several use the non-standard <noindex> element, which is used by the local site search engine.
Styles are mostly CSS2. I also use a few from CSS3 when I am confident they provide no problems for display in browsers which do not support them (e.g. border-radius).
As for accessibility, the general aim is to conform to level AA of the Web Content Accessibility Guidelines, though I wouldn't necessarily claim that all pages have got there yet.