Just because I like to shut people for life: Advertisement and decoration, you don't need an HTML element with title "pen for background decoration" where you want people to focus in your product. The other one is separation of concerns, many times the programmer and the designer work together, but is sometimes unhealthy to give the designer access to the templates (or php, whatever), is way less dangerous to give him access only to CSS, so using stuff like this helps in such case.
To say nothing of the fact that, because you can embed images in CSS, this sort of thing is only interesting at all if you actually look at the CSS source. It's kind of like someone making scrambled eggs with their hands tied behind their backs. Sure, it's challenging, but why should anybody care?
I don't want to discourage people from playing around with CSS, or even from trying to make scrambled eggs with their hands tied behind their backs. It's good for people to challenge themselves in all kinds of creative ways. But this is supposed to be Hacker News. Making icons in CSS hasn't been newsworthy in a very long time, and I think it's lamentable that something like this is at the top of the home page while vastly more interesting things (like https://news.ycombinator.com/item?id=8224972) languish in obscurity.
I don't want to discourage people from playing around with snarky comments, or even from trying to make people feel bad with their hands tied behind their backs. It's good for people to challenge themselves in all kinds of creative ways. But this is supposed to be Hacker News. Making hypocritical comments hasn't been newsworthy in a very long time, and I think it's laudable that something like this is at the bottom of the comments page while vastly nicer things (like https://news.ycombinator.com/item?id=8229841) revel in the limelight.
It might be hard to realize that not everyone who visits the site has been around as long as the site has existed. There's nothing wrong for old content to be revisited; either for new eyes or refresher for old.
Plus, I guess I'm sorry the masses don't find some articles as fascinating as you?
Show me one instance where this is actually applicable and more semantically appropriate than embedding an SVG and I'll shut up for life.