Comment by onion2k

It doesn't work properly. The body has a max-width of 750px, and the header uses a padding hack of 'calc(50vw - 50%)'. That 50% means '50% of the containing element', so if 50vw is more than 750px (eg your viewport is more than 1500px wide) the header element stops being centred and starts spilling over on the right hand side.