As far as I can see, cloudflare (the CDN we are using) is accessible from China: http://www.greatfirewallofchina.org/index.php?siteurl=https%3A%2F%2Fcdnjs.cloudflare.com%2Fajax%2Flibs%2Fsemantic-ui%2F2.2.13%2Fsemantic.min.css
HTTPS links to cloudflare do not seem to be working, though, and that's what we are using: http://www.greatfirewallofchina.org/index.php?siteurl=http%3A%2F%2Fcdnjs.cloudflare.com%2Fajax%2Flibs%2Fsemantic-ui%2F2.2.13%2Fsemantic.min.css
The failure looks more like a technical one than obvious censorship like this one, though: http://www.greatfirewallofchina.org/index.php?siteurl=https%3A%2F%2Fgoogle.com

So it's a bit odd... I don't know what to make of it.

In any case, once we add a cache in front of our website as Sanne Grinovero was planning to do, we should be able to host the JS/CSS files ourselves. But maybe this cache is already set up ?