It's a lot easier to implement this on the server side than to try to get compatibility between the various browsers (especially Internet Explorer). A server-side implementation is also compatible with older browsers, which is a concern that would prevent CSS variables from coming into common use for a long time.
It's not just about the effort for browser developers. It's also about the effort of getting all the browsers in use to be more-or-less compatible with the standard.
Comments
It's a lot easier to implement this on the server side than to try to get compatibility between the various browsers (especially Internet Explorer). A server-side implementation is also compatible with older browsers, which is a concern that would prevent CSS variables from coming into common use for a long time.
It's not just about the effort for browser developers. It's also about the effort of getting all the browsers in use to be more-or-less compatible with the standard.