Not sure why you are being down-voted, as this is very much true. If you can't comment on CSS on HN, where can you?
Sure, CSS is on-topic on HN, just like this article about cryptography is on-topic. However, commenting on the HTTPS certificate particulars of a site with a blog post about CSS would be similarly off-topic in a discussion about that blog post :)
(Personally, I am on the fence about the appropriateness of the comment, but I hope I have shed some light on why others would down-vote it)