As you know, there are a number of companies producing a number of different web browsers. So who sets the standard for what is and isn't official HTML? There is an organization that does this called the World Wide Web Consortium (W3C). They are an international consortium that sets the standards for HTML. So if I was a company that was going to make a web browser, I'd go to their site and find out what the standard was and make sure that my browser could read and interpret everything in the standard.
And all decent browsers should do this at the very least. The problem arises when those companies add their own tags that only their browsers can read, or make a web authorship program that tosses in extra and unecessary tags and/or uses these company-specific tags.
The moral of the story is, that if you write your own HTML and make sure that you only use tags from the W3C standard set, then every web browser will be able to read your content.
Here is the W3C website:
HTML continues to evolve and grow just like everything else that has to do with computers, and there are several versions of HTML. Since LPL is an educational institution and I feel that we need to make the content of the LPL web pages available to the widest variety of people, I use HTML 3.2 to write the Department web pages (this one included).
HTML 3.2 is old, it was put into use in January 1997. However, this means that even relatively old web browsers can read it. That way users don't need the latest, cutting edge web browsers just to look at the LPL pages. The version of HTML that you choose to use is up to you, but keep in mind the target audience that you're shooting for.