How it renders a html/css/js and many more?

Actually all the browsers exists today are using some “layout engine“.
Yes this what keeps running these browsers.

A layout engine, or rendering engine, is software that takes web content (such as HTML, XML, image files, etc.) and formatting information (such as CSS, XSL, etc.) and displays the formatted content on the screen.

A layout engine is typically used for web browsers, e-mail clients, or other applications that require the displaying (and editing) of web contents.

There are lots of “layout engines” like Gecko, Boxely, GtkHTML, HTMLayout, Trident, WebKit etc. this is a long list. Some of them are open source and some are close source

Lets look at who created what and what is being used by which application.

Gecko is known to be most powerfull layout engine after Trident by microsoft.
But i think Gecko is the most popular/powerfull as Trident works only on windows.

Gecko offers a rich programming API that makes it suitable for a wide variety of roles in Internet-enabled applications, such as web browsers, content presentation, and client/server.

Mozilla uses Gecko for most of its application like firefox, thunderbird etc …

h