A standard is whatever goes into https://html.spec.whatwg.org . Period.
CSS Flex is a standard. If Konqueror, Firefox, Edge, or whatever other browser doesn't support it (I know they support it now, but that wasn't the case 2 or 3 years ago), it's not any of my business. I'll keep using Flex because it's an accepted standard. And my webpages will work on any browser that supports that standard. If that means that my website works only on Webkit-based browsers, well, then that's really not up to me to fix, isn't it? That is something that the people behind other browsers should fix, by implementing "Flex".