Marc's final diplomatic reply:
"So, we're probably going to go with <IMG SRC="url"> (not ICON, since not all inlined images can be meaningfully called icons). For the time being, inlined images won't be explicitly content-type'd; down the road, we plan to support that (along with the general adaptation of MIME). Actually, the image reading routines we're currently using figure out the image format on the fly, so the filename extension won't even be significant."
That's a good lesson to recognize and appreciate.
Rather than focus on stuffing new tags into HTML, we should focus on pushing the abstraction level lower. I'd really like to see something along the lines of a secure/safe <script language="LLVM"/> and an analogously low-level rendering stack.
In implementation, it's even worse: the Content-Type in nearly every HTTP Response on the web is either text/html (default, so it triggers sniffing), application/octet-stream (because your webserver doesn't know any better), or sniffed by the webserver from its extension in the filesystem (again, FUCK ME).
On the client-side, mouthbreathing open-source developers are always paying ultimate fealty to Content-Types that were naively sniffed on the server -- text/x-python? how could I possibly display that?
application/octet-stream has had to be used for things like .zip files because Internet Explorer insisted on opening up application/octet stream even the browser was configured to save it. And to make matters worse, there were some configurations where Quake3 .pk3 files, which are actually zips, wouldn't save by Internet Explorer, so you actually needed to rename them to .pk3.zip, which would sufficiently confuse the browser and prompt to save it.
Using a text/* renderer.
Sadly, Firefox doesn't handle this properly natively. Happily, there's a plugin for that: https://addons.mozilla.org/en-US/firefox/addon/8207
Why aren't Mozilla using Apple's "There's an app for that" to push "There's a plugin for that" as a way to counter the rise of Chrome and Safari?
It seems piggy backing off Apple's propaganda that quantity > quality = best would be an easy thing to do here.