This post on StackOverflow explains things pretty well.
Also, I'm moving this post to the Photoshop General Discussion forum, whose participants may want to chime in.
Product Integration Manager, Creative Cloud Learning & Training
Standard Image file format may have different capabilities in many cases. Example Png supports transparency Jpeg does not. Png supports 8 and 16 bit color depth jpeg only supports 8 bit color. All png files are not the same there is some for some thing may be optional. I believe the PNG file format standard support an Alpha channel but one is not required. Adobe has a Photoshop plug-in that supports png files there is also a third party plug-in that supports png as well known as supper png. There is also pgn files that have animation. Not all browsers support them and only display the first frame. Photoshop can not save animated png files. The following animated png may not animate in your browser. Jpeg file have a single photpshop background type layer.