Roger, I only dabble with HTML. I haven't got time to search right now but there is an HTML standard (in fact more than one as HTML has been "upgraded"). All browsers designed to adhere to that version of the standard should behave in a consistent and defined manner but anything outside such as Netscapes blink, IE's marquee may or may not work in another browser.The same applies to errors in HTML. It is down to the browser to decide how to handle them.
With regard to the versions, as an example, older browsers do not support frames and these issues need to be considered when designing a web site. Personally, I would err towards compatibility and I think that is what Max does with Mudcat - IMO it is better to have something relatively simple that is accessible to everybody than to use all the latest all singing and dancing features and find that you are excluding a large number of people who would like to access your site.
One final comment, if you ever do attempt to design a web site, test it on whatever browser you have available before putting it on the web. It is easy to make a mistake that will not show up on one browser but will cause problems or not work with another.
Jon