A large number of websites, instead of storing CSS, images, Javascript, and other similar static content on the same server that processees user input (like navigating pages that change if you login), they often choose to store them on a separate server called content delivery network (which may be distinguished by having CDN somewhere in the hostname). Server computers may go offline due to maintenance or denial of service attacks, as such, you may see a website working fine but not see the CSS and images. The CDN server simply went offline