This is a really awesome thing that 10Up did for the community. As it is said, this doesn't only have to do with WordPress itself, but legacy compatibility as a while. I just had a site where I spent about 10 hours doing back-compatibility for IE8 becasue flexbox doesn't work on it; creating fallbacks using Modernizr. If this was open sourced a few weeks ago I would have been so much happier. Great work 10Up!