That's exactly what I'm working towards. It's the difference between just using an XML file and using an XML file with a unique extension tacked onto it. It rep

That's exactly what I'm working towards. It's the difference between just using an XML file and using an XML file with a unique extension tacked onto it. It represents a subset with set expectations for behavior. That expectation is what matters. For generic containers, there are no expectations on what's inside. If I'm making a video player, do I support MKV? Well, some of them. Not all of them. You have to try it and find out. Heck, even within a single codec it's hard to find complete support. Last I checked, browsers won't touch an H264 stream unless it's also YUV420p which also places certain restrictions on physical dimensions of the data and probably some other stuff that's escaping my mind. If I'm making an image viewer, do I support SVG or PNG? For this example, let's say I only support PNG, but I can tell from the outset what's inside any file I might try to load. I can confidently say I can meaningfully load and display anything that uses the established standards of the PNG format to store image data. Sure, there are extensible bits, but they're not part of those core expectations surrounding the core part of the format, the image data I care about. And I didn't have to try and open the PNG first to see if it was actually an SVG. I can recognize not-bitmaps at a glance without having to inspect them any deeper than their extension.