That one is not that complicated if you don't think about the math. It's basically just if we interpret the float as int and add a magic number we have a good estimation.
From what I remember at least, it's been a little while since I implemented it.
On the other hand, it made webpages way less flexible.
Like yesterday (i have the browser not in fullscreen, for reasons) on my 16" fullhd notebook, webdev couldn't imagine that someone would use his site in a ~1000px browser window, sidebars left and right, the main content about 20 characters wide squeezed inbetween. So i pressed f12 and deleted the sidebars. But the content was still 20em wide, because of flexbox.