How to setup html page for test

I have a pretty big project without any test coverage and it starts to backfire. But I am new to testing and don't really know where to start.

I already set up degradation tests with PhantomJS. It works very well but slow: I need to build whole project, load it to browser and wait for hundreds of screenshots to be done.

So now I am looking for way to test each of my modules separately. Problem is — I don't know how to properly set things up.

My project running in two steps: first, at compilation time, I am building static HTML with modules; then, at runtime I am modifying this modules to match browser environment.

So, my problem, I guess: I need to build some test page, compile it, load it to browser with runtime scripts and meausure dimensions of resulting elements or something else.

Where do I start? Or maybe I just missed some core concepts and need to read something first?