A couple of years ago, Joe Gregorio explained Why so many Python web frameworks? and showed how to create your own Web framework with a few lines of code. The most fundamental bricks are packaged in the standard python library.
There are always been many hypothesis about why the Web was successful, all of them can’t be verified, because we can’t restart the experiment. But retrospectively, it is interesting to look at what has been reused widely. libwww is a library including all modules to create a Web application, such as http, html, etc. It has been initially written by Tim Berners-Lee, and was freely available. People could just take the library and put an interface on top of it to make a browser, a Web server, an indexing bot.
One of the very difficult and technical parts of a browser is the layout (or rendering) engine. It takes the Web content and displays it after having processed the style and scripting information. Some of these layout engines are open source such as WebKit, Gecko and KHTML. Others are sometimes sold to third parties for developing specific products. Web developers take them and create new browsers or use them in their applications. For sure it is a tad more complicated than just taking the libraries but it became “easier” to create a browser. It is known as BYOB.