[hinkley]

PNG solved this problem when BGP was still young: each section of an image document is marked as to whether understanding it is necessary to process the payload or not. So image transform and palette data is intrinsic, but metadata is not. Adding EXIF for instance is thus made trivial. No browser needs to understand it so it can be added without breaking the distribution mechanism.

[zajio1am]

This is also how BGP (mostly) solved it. Each attribute has 'transitive' bit. Unknown attributes with 'transitive' bit set are passed, one without are discarded.

[immibis]

... Except for acTL, which is a special exception because it turns out that wasn't sufficient to ensure consistency in 100% of cases.

[hinkley]

I was never that enthusiastic about motion PNGs in the first place. We have so many other ways to achieve that now.