What's even more impressive than Sam's web site (which has been online for years) is that he and his two Hong Kong-based co-founders bootstrapped Bitmex, a multi-billion dollar company that handled over US$1 trillion of trading volume over the past year with almost no investment.
It seems to use the prefix for the browser you're visiting the site in (I see -moz- properties). Fine, but I'm not convinced the prefixes were required at all for quite a few of the properties.
It's legacy. This dates back to April 2015 when there were still some reasons to use prefixed properties. Rather than list all of them in css - which would have been ugly - I choose the one useful to your browser.
I thought it was gonna be like an experiment I did a while ago, a webapp that can edit and deploy itself. Ended up with prototype[0] but would love to see the possibilities in a more polished version, and thought this submission was it!
https://no-gravity.github.io/html-quine/index.html