Computer science is a very broad subject. There cant be a direct contrast of cs and web development. Web development is one of the area that involves computer software, networking, machines, protocols and many more i don't know of.
Web development is not entirely limited to css, html and a bunch of images on a website. Web developers mostly are programmers and they can code both for almost all platforms. There is no limit to what a programmer can do with machines. For instance bitcoins is implemented in c++ and has good enough algorithms to make a difference in the world.
computer scientist! bleh!!