I agree with you about it's being one of the best formats both for size and features.
And I'm well aware that Livecode is not a web browser or web authoring tool.
But internet gives direction to the computing era.
If consortiums like W3C doesn't accept a technology as standard,
browsers don't likely support it { except IE, it doesn't support anyway

};
if browsers don't support, end-users won't aware;
if end-users won't aware of or interest in a technology, the developers won't bother to code {Windows Phone?};
those developers {us} who develop apps targeting end-users
and those developers {adobe, oracle, etc.} who creates authoring/creating tools.
This leaves the company alone who tries to support that underrated technology.
It's hard to support a technology all alone.
Coding and maintaining libraries/frameworks of this scale is hard and requires vast resources.
Even Mozilla community has secondary thoughts about
apng nowadays, have a look at bugzilla.
RunRev's developers are next to nothing in numbers when compared to Mozilla.
That's why I wouldn't put much hopes in this.
Svg is another story; it's being already actively developed by hundreds if not thousands of developers.
So, all RunRev has to do is finding a way to integrate it into the engine
and keeping it neat, maintaining, updating, fixing minor consistency issues.
As I said before, I'm a major fan of
apng for quite a time;
but there are facts.
Unfortunately, being better than others doesn't always result with success.
Anyone remember BeOS?
What an excellent piece of code it was, way ahead of its time.
NextOS was just bloatware compared to it, in my opinion.
However, it was NextOS who succeeded and came to these days.
How about Hypercard?
Adobe's blackmail of "pulling Director support from Macs" to Steve Jobs killed it, well almost

Thanks to its predecessors, we are still benefiting its innovations.
S*ck it adobe
Sorry for belaboring but that's because I really loved apng and wanted to see it's been supported widely.
All the best,
~ Ender