How do web browsers work

Under the hood: this is how browsers work

Thorsten Eggeling

For many users, the browser and the Internet are practically identical. But what exactly happens when a website opens, and how do the pages get into the browser?

EnlargeYou can hardly do without a browser in everyday life. However, one seldom wonders how these actually work.
©ünay Mutlu

Web browsers are the most widely used programs on the PC today. They provide information on any topic, are used for communication in social networks and enable online shopping and online banking. More than 20 years ago it was not foreseeable that the browser, and with it the Internet, would develop in this way - the idea of ​​the World Wide Web was born back in 1989.

First browsers and the "browser war"

The first popular browser, NCSA Mosaic, was able to display images in its own window in 1993. Who would have thought that such an inconspicuous program could become the central software on all internet-enabled devices? A year later, the head of the development team founded Mosaic Communications Corporation, which was renamed Netscape Communications Corporation shortly afterwards. However, Mosaic was not the very first browser: As early as 1990, Tim Berners-Lee published a browser at Geneva’s CERN under the name “Worldwideweb”. However, it only ran on less common Next-Step computers and could not display any images. The Netscape Navigator remained the main browser until Microsoft discovered the Internet. Its Internet Explorer was poorly programmed and disregarded applicable standards, but it came together with Windows. For many users, the blue “e” on the desktop has become the standard route to the web. At least in Germany, however, Netscape won the “browser war”, because Firefox has the largest market share here. The Mozilla Foundation was founded by Netscape, and the Netscape source code forms the basis of today's Firefox.

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EnlargeNCSA Mosaic was the first popular browser. To this day, nothing fundamental has changed in terms of the functions. there is still an address bar and a couple of buttons for navigation.

The World Wide Web as a basis for communication

Today we open sites from all over the world, download files and watch videos. Such data exchange is neither trivial nor self-evident: Anyone who has ever tried to open text documents with the "wrong" program knows this. Tim Berners-Lee faced similar problems in Geneva in the 1990s. The data transfer between the French and Swiss laboratories at CERN was difficult due to the different network infrastructures and stood in the way of the exchange of knowledge. But Lee wasn't just interested in exchanging data. He wanted a freely accessible knowledge network in which everything is linked to everything. And everyone should be able to contribute their own knowledge.

For such a network you need the following ingredients:

• a markup language that structures and links information

• an editor with which the texts can be created • server software (web server) that makes data available in the network

• Client software (browser) that runs on every PC and can call up content from the server

Lee soon created the necessary components, especially since he didn't have to reinvent the wheel: a text markup language could be developed on the basis of SGML (Standard Generalized Markup Language). (S) GML was created as early as 1969 to structure books into chapters, headings and sections. Elements in the form of " ... " are used, which are referred to as tags (label, marking). Berners-Lee added an element for hyperlinks, which can be used to link documents and images, and therefore called the whole thing HTML (Hypertext Markup Language). Since HTML is stored in simple text files, there are no special requirements for an editor.

The server, on the other hand, was more difficult to develop, because beforehand the network protocol HTTP (Hypertext Transfer Protocol) had to be defined, which regulates the communication between HTML / web server and client (browser). Something similar already existed - and still is today - with the name FTP (File Transfer Protocol) for the transfer of files. Finally, Lee only needed a program that retrieves HTML files from the web server via HTTP and prepares them in a readable manner.

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EnlargeWith a look at the HTML source code, savvy web users can explore the inner structure of a website. The browser converts the HTML tags visually in the angle brackets.

This is how a web browser works

Little has changed in the basic structure of a browser to this day. The content of the website appears in the window, the user clicks on text or image links and gets to other websites. "Forward" and "Back" buttons can be used to scroll between the pages called up, and a home button takes you back to the start page. Hence the term “browser”, which is derived from “browsing”, which originally referred to navigation in text files using buttons such as “forward” and “back”.

A page is called up by entering a web address in the form (URL, Uniform Resource Locator) in the address line and confirm with the Enter key. "Http: //" can be omitted because the browser adds it automatically. If you want to download files from an FTP server, put “ftp: //” in front of it. Even the “www.” Is now actually dispensable, because under most Internet domains only one web server is accessible anyway. From a technical point of view, “www” is a sub-domain. This enables providers to call up “” with “” or “www.domain. de “to deliver different pages.

What happens in the background: Calling up an address, whether by clicking on a link or by entering it directly, initially leads to a request from the DNS server (Domain Name System). This knows the IP number of the Internet domain called up and communicates it to the PC or DSL router. Communication between the web server and browser can now take place. The DSL router acts as a converter for the public IP number that you received from the Internet provider and the IP number of your PC in the local network. The browser uses the Hypertext Transfer Protocol (HTTP) to request the server to send a web page. If only the domain name was specified in the form “”, the server sends its start page. If a path is attached to the URL, for example “”, the server delivers the requested file. If this is not available, the server returns a 404 error ("Page not found").

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The browser saves the delivered HTML file with all elements in a buffer (cache) on the hard drive and then loads them from there. Elements that are used more than once do not have to be reloaded via the network. Each HTML file is read from top to bottom and the structure is interpreted (“parsed”). The result is a data structure that the browser visually converts into text with formatting, colors and images in the correct positions ("render").

Today's web pages are not just made up of HTML code. There is also CSS (Cascading Style Sheets). The HTML file contains the logical structure of a text with headings and paragraphs, CSS takes care of the formatting, i.e. different fonts and sizes, the colors and the positioning of text blocks and images.

Javascript, which the browser also has to interpret, ensures the dynamism of websites. HTML or CSS elements can be changed via Javascript after the browser has already displayed them. This is used, for example, when you click a button, a database is queried in the background and the result then appears directly on the website. Without Javascript, the browser would have to reload the website, which is laborious. This technology enables complex web applications such as text or photo editing.

What browsers cannot do themselves, plug-ins such as Flash, Java or Adobe Reader do. These are external programs to which the browser only forwards the content. Plug-ins are therefore something different from internal add-ons or extensions that supplement the browser functions.