The short answer is that there is no right HTML standard. If it validates, browsers should display it. Now, the longer answer.

Currently, there are eight different HTML “standards”:

1. HTML 2.0, or RFC 1866, is technically obsolete,
2. HTML 3.2, the W3C’s first HTML recommendation,
3. HTML 4.0, which has been superseded by…
4. HTML 4.01,
5. ISO HTML, or ISO/IEC 15445:2000 (a subset of HTML 4.0),
6. XHTML 1.0, which mixes HTML 4.01 with XML,
7. XHTML Basic, which is for devices like PDAs,
8. and XHTML 1.1, which is like XHTML 1.0 but uses modules.

Another version, XHTML 2.0, is still a working draft.

Despite the long list, choosing one is fairly easy.

• First. you can ignore the first three because they are outdated, and the vast majority of the browsers in use support HTML 4.01.
• If you think you may one day use XML, or want to create a more robust document, or one that relies on the DOM and CSS, use a version of XHTML. Given XHTML 1.1 should be served using a special MIME type that is not recognized by many browsers, XHTML 1.0 may be the better choice.
• Only use XHTML Basic if the site is intended to be viewed on a small device.
• If you’re still looking for a recommendation, it will come down to either ISO HTML or HTML 4.01

Both ISO HTML or HTML 4.01 are fine to use, as both are standardized HTML specifications, however HTML 4.01 is by far the more common and is generally considered the “Web standard.”

ISO HTML, which was mainly developed by the W3C, is a reformulation of HTML 4.0, but is a bit more rigorous. For example, an <h2> must be preceded by an <h1>. (The reason ISO HTML even exists is that the ISO recognized HTML as a de facto standard that warranted official standardization. By doing so, it assumes HTML will be around for quite a long time and will be managed by W3C.)

So, to summarize, for most Web pages use HTML 4.01 or, better still, XHTML 1.0.