Comment by myfonj

I wish external entities were enabled in browsers (*), and not entirely disabled, like they are now.

In the OP scenario, it would provide possibility to have header/footer/stylesheet content in separate files and get system working that does not rely on absolute path of the CSS.

(*) Naturally, just with few minimal directory traversal precautions; basically anything like "no ../" and even restriction like "referenced resource filename must begin with referencing XSL filename" would be fine for me.