It’s my understanding that websites can identify your OS and there’s nothing we can realistically do about it, but it has something to do with how fonts are rendered. I’m not an expert, but this is what I recall from trying to obscure my OS from websites.

Here’s a post on the matter: https://security.stackexchange.com/questions/173233/how-do-i-hide-the-os-i-am-using-from-internet-sites

Javascript isn’t that big of a deal.