This is more of an issue with Safari than anything else - Apple's slow speed at keeping up with web standards is reasonably well documented (http://nolanlawson.com/2015/06/30/safari-is-the-new-ie/) (note: this article generated a fair amount of controversy when it was originally posted on HN)

I've just tried this site in the very newest OSX Safari and it looks dreadful. Disappointing.