The web isn't video games, where it's well-understood that certain games only work on certain systems. The web is supposed to work everywhere. And not just in a fancy smart people philosophical way. In the sense that regular users expect that the sites they visit work in whatever browser they prefer, are used to, or are required to use. They'll assume the site is broken if something doesn't work as expected.
CodePen users are damn smart with tech stuff, that's the nature of our business. But still, we still assume sites are broken when they don't work right in our favorite browsers, forgetting that it might be a browser compatibility issue.
We do have a lot of data suggesting that CodePen users are extremely modern in their browser choices and also extremely desktop focused. We've made, and continue to make, choices around that data. Things like what technology we use to build the site and how far back we're willing to go with support.