Until it is approved, there is no HTML 3.0 DTD, there are only proposals. Since the common model for software standards is to standardize existing practice, there can be no new standards unless people develop new practice for standards groups to later adopt. Especially in a rapidly developing area like HTML, any vendor who is well-tuned to her customers is going to need to extend the standard to meet specific market needs. I assume the Netscape people are participating in the specification of the next version of the standard. I have no doubt that when the standard is adapted, Netscape will render conforming tables correctly and that they will still have extensions providing additional features that their customers like.
Since a great fraction of the net has voted with their dollars and their downloads to say that Netscape meets their needs better than other browsers, it would be pretty foolish for Netscape to modify their browser in a way that made it support fewer of their customers' needs.
In the meantime, "supporting HTML3" tables is an untestable assertion useful only as a statement of intent. I have seen a draft of HTML3 that seems to fall within the set of tables that Netscape can render correctly. The latest draft has added some wrinkles (header/body distinction) that they don't support. That's the way it is in the world of developing standards.
-- scott preece motorola/mcg urbana design center 1101 e. university, urbana, il 61801 phone: 217-384-8589 fax: 217-384-8550 internet mail: email@example.com