A 13px font for paragraph text is nearly hostile. It's not that legible to people with perfect eyesight, but then it's not at all legible to anyone with imperfect eyesight. It's like saying you don't care if anyone who reads your blog would struggle doing that. And given how very simple it is to change, it's kind of insulting, specifically given how many years usability has been a thing.
10 years ago I wouldn't have written this comment. But now this isn't how you behave if you have an audience.