I won't necessarily critize you too much on your second footnote :-), however...

I don't have a problem with a browser maker adding new tags to try to differentiate themselves from the competition. However, three significant points must be made:

1. We have yet to see a fully-compliant browser. When both NS and IE were getting new tags, neither supported the HTML3 or 4 spec exactly, and in the case of some test suites, they were less than 90% compliant with standards that were well-established (6 or more months) from release. If you cannot get the standard right to begin with, you really have no 'right' to start adding new features. This is most dramatically shown when you talk about CSS and IE3 or NS4, as well as the introduction of TABLE in NS. I'm still waiting for a browser to adquetely impletement the OBJECT tag that is part of HTML4 such that one can provide multiple levels of alternate content for multimedia elements.
2. Most of the 'new' tags were ill-defined from an SMGL stand-point, in that in browsers where the tags were not supported or impossible to render, there was no way to deliever alternatively content without significant workups behind the scenes or hacks. Again, TABLEs come to mind specifically with when Mosiac and Lynx were significant, as well as IE's MARQUEE tag, or the IFRAME tag.
3. A 100%-standards compliant browser should be able to view the majority of the web (ignoring plug-in technology) without problems as long as the addition of new tags to standards (or non-standards as the case may be) follow standard SMGL rules; even today, a browser with the ability to use DTD's effectively will be able to handle new additions to the HTML/XHTML specs without problems. As it is, no browser yet can claim that, and those few rendering bug differences between the latest-and-greatest make it very hard to design good pages that look right on all browsers, notably due to slight differences in CSS2 support and other newer features.

As someone who's been using the web since nearly day 1, I've been rather disappointed with how the browser wars initially fractured the design of web pages, but most of those wounds are healing with the latest browser releases; it's very hard to find pages that "You must be using Browser X to view this page" save for antiquited pages. Right now, if anything, the splintering of the web is mostly due to using closed-vs-open formats for media (GIF vs JPG/PNG, Real vs Windows Media vs MPeg vs Quicktime, etc). But I still believe that all browsers should strive to get 100% of HTML4 and CSS2 before they move on to other arenas; deliver that to us, and designers would be overjoyed that they can actually write once, view anywhere once again.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
It's not what you know, but knowing how to find it if you don't know that's important