`font-size: calc(1.25em + .5vw)`. Please never, ever do anything like this.
On a 1920×1080 display, given a browser em of 16px, that would be 29.6px. When practically every other page around will be in the range 16–22px (mostly 18–20px, I reckon). It’s 30–70% too large.
On a 400px-wide phone with 16px browser em, that would be 22px. When practically every other page around will be in the range 16–18px. It’s 20–30% too large.
On my 1706×960 laptop display, it’s 28.5px, and has the dishonourable distinction of being the first site that I’ve actually zoomed all the way out to 50% for, and not felt any remorse at all, because that makes the text effectively 18.5px for me, right about where I reckon is right. I’ve never felt the need to go past 67% before. If it weren’t for the fluid typography, I wouldn’t have even had to, but because the viewport width is such a large part of the calculation, it decreases the effectiveness of the browser’s zoom, so the 50% is actually more like 65%. This also means that things like changing the browser’s zoom will throw you around the document willy-nilly, because it’s working against the browser’s techniques to keep you in the same place in the document.
What it talks about has a clamp() invocation to prevent values from getting too stupid, but the site itself doesn’t have that, and starts already oversized.
This is a real pity, because there’s a lot of good about the content itself—in particular, its remarks on how people accidentally make calculations assuming the browser em is 16px are spot-on. But the content is immediately compromised by the page’s crazy font sizes.