Does validated HTML code guarantee that a page will display correctly?

Unfortunately, no. Browser manufacturers don't always code for valid HTML - especially in pre-release or beta versions of browser.

All "final" versions of browsers should display a valid HTML page without any problems.

A good example is Internet Explorer - this browser can have bugs that get fixed, which re-appear in a later version.

Example:

Internet Explorer 5.5 SP1 fixed many CSS problems, plus fixed a LOT of SSL problem (Secure Socket Layer - for e-commerce & secure transactions). The big problem came when IE 5.5 SP2 came out - almost all of the bugs fixed by the SP1 patch were wiped out! Fortunately, Internet Explorer 6.0 came out only a few short months later - the CSS bugs were largely fixed - but the SSL bugs remain to this day, something which web servers must accommodate and fix on the server side for all IE 6.0 equipped visitors!

Related Topics:
What is involved in marketing a site?
What are the potential pitfalls of a site that doesn't validate?
Is W3 Validation essential for a page to display properly?