Hi, now most of website should be "secure", so it could be great to acces to your file w3.css with the https protocol. It could allows us to have a file auto-update without have to download it for include it in our website.
The actual link is : http://www.w3schools.com/lib/w3.css
Get the same with https could be an asset.
(sorry for my english if it's hard to read me...)