Taking advantage of the way a browser renders a very thick border is a tremendous hack, with all of the attendant admiration and alarm the word implies.
This hack is clever, but an HTML5 canvas would be an easier to implement and more straightforward solution.
Indeed, there are other technologies that can build a digital clock more easily, but I think the point was to further demonstrate the versatility of CSS. I've personally never before seen CSS do something like this. I also enjoyed how the author took the time to explain how div borders were used to create the numbers.
If that was new to you, then this should blow your mind (it certainly did mine 3 or 4 years ago). It uses roughly the same border trick to draw a 3d rotating polygon.
That's impressive! I had no idea borders were this versatile.
Following the easier path isn't what got man on the moon. It's people like this that demonstrate lateral and creative thinking that is exactly what the computer industry needs to thrive.
I'm curious what the easier path to the moon was, and why they didn't take it.