I've not seen a browser in quite some time that couldn't view PNGs. And, to be honest, as much as HTML tries to be backwards compatible (even if the authors circumvent that as much as possible), there are so many new technologies in web pages these days that a browser that can't display PNGs probably can't display a lot of other stuff, either.

MNG, on the other hand, is still widely unimplemented, and animations are the only area where GIF holds the upper hand, but, even then, solely based on wide availability.

And JPEGs aren't intended to compress graphics. It's ``Joint Photographic Experts Group'', after all.

But neither format is proprietary. Proprietary means that the format is not documented and/or is prevented from being freely used by legal means. Both JPEGs and PNGs are non-proprietary. Lack of wide usage (if that were the case) doesn't change that fact. It makes them less useful, but that's a different story.