There's a large number of tools that expand the options available to you as a front end developer (as well as tools that are part of the ecosystem that supports the first set of tools). Basically all of those tools are built around the assumption that you can just use javascript.

You can build sites that function without that, that have javascript as an optional thing that spruces up the site for people who use it. But you will be putting in more effort. ReactJS (which Lesswrong 2.0 is built on), is built around the premise that you literally just use java... (read more)