https://news.ycombinator.com/y18.gif
It's 0.1KB. It's so small that any SVG version of it would actually be a larger file (seriously, I tested it with an SVG converter script online, and the SVG came back at 0.6KB).
There's no practical reason to convert it, at least not as far as file size is concerned.
It's about focusing on interesting content rather than pixel perfection.
It's about shipping on Tuesday rather than shipping on Friday.
It's about optimising the things that are important.
I like it, even if it's a big pixel-y on hi def displays, it's a reminder that done is better than perfect
But then again, HN itself is already easily fast enough, and you don't optimize when you don't have to.
But probably not worth the effort - the extra CSS might mess up the equation.