Functions of a web engine?

I was reading this post about WebKit/Blink and they mention V8, Google's JavaScript engine. So if the web engine has a separate engine dedicated to enacting JavaScript, is the web engine itself used to parse/render HTML tags & CSS? Is it used for anything else?

Like in Rails, if I'm not mistaken, the content served by the web server has the embedded language (Ruby) already evaluated and what's sent is proper HTML/CSS/JavaScript. Is my understanding correct?

Thanks