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Foundations

Web browser

Summary

A web browser is a program that fetches pages from the web, turns their HTML, CSS and JavaScript into what appears on screen, and lets people follow links. The first, WorldWideWeb, was written by Tim Berners-Lee at CERN in 1990. Today the major browsers run on three rendering engines: Blink (Chrome, Edge), WebKit (Safari) and Gecko (Firefox). As of September 2026, Chrome had 66.36% of page views worldwide by StatCounter's count.

Web Building Encyclopedia · Foundations · Last reviewed October 22, 2026 · 11 min read

What a web browser does

A web browser is a program that fetches pages from the web, displays them and lets people reach other pages by following links. MDN, Mozilla’s developer reference, calls the browser the most familiar type of user agent, meaning software that acts on the web for the person using it.1

Opening a page takes several steps, which MDN’s guide to how browsers work sets out in order:2

  1. Find the server. The browser asks the Domain Name System (DNS) to turn the site’s name into an IP address, the numeric address of the server. Each separate host name a page uses needs its own lookup.
  2. Connect. It opens a connection to the server with a TCP three-way handshake. For a secure HTTPS address it then negotiates TLS, which picks the encryption method and verifies the server before any page data is sent.
  3. Request the page. It sends an HTTP GET request, most often for an HTML file. MDN notes that the first chunk of the reply is usually 14KB, so the HTML and CSS needed for the first view should fit in it where possible.
  4. Parse. It turns the HTML into the DOM (Document Object Model), a tree of every element on the page, and the CSS into a matching tree called the CSSOM. A preload scanner reads ahead and starts downloading style sheets, scripts and web fonts early.
  5. Run scripts. JavaScript is parsed, compiled and run, most of it on the main thread.
  6. Build the accessibility tree. Screen readers use this semantic version of the DOM, and until it is built the content is not available to them.
  7. Style, lay out and paint. The browser combines the DOM and CSSOM, works out the size and position of every box (layout), turns the boxes into pixels (paint) and, where parts of the page sit on separate layers, stacks the layers in the right order (compositing).

MDN says browsers are, for the most part, single-threaded: they finish one task before starting the next. For scrolling and animation to look smooth, work on the main thread has to take less than 16.67 milliseconds per frame, which is why a page loaded with heavy scripts can feel slow to respond.2

JavaScript runs in its own engine inside the browser. V8 is Google’s open-source JavaScript and WebAssembly engine, written in C++ and used in Chrome and Node.js.3 Apple’s JavaScriptCore began as a fork of KJS, the JavaScript engine of the KDE project.4

A modern browser is also several programs working together. In a 2018 series on Chrome’s design for Google’s developer site, Mariko Kosaka described a browser process that runs the address bar, bookmarks and back and forward buttons, renderer processes that draw what appears inside each tab, and a GPU process that handles graphics work. Since Chrome 67, desktop Chrome has given each cross-site iframe (a page from another site embedded in the page) its own renderer process by default, a feature called Site Isolation.5

Rendering engines

The core of every browser is its rendering engine, also called a layout engine or browser engine: the software that interprets HTML, CSS, JavaScript and images and draws them on the screen. Mozilla’s description of its own engine, Gecko, shows how much the word can cover: a networking stack, a graphics stack, a layout engine, a JavaScript virtual machine and porting layers that adapt it to each operating system.6

The engine and the browser are not the same thing. The browser wraps the engine in the address bar, bookmarks, buttons and settings that Kosaka’s article calls the “chrome” part of the application.5 Several browsers can share one engine, and one browser name can use different engines on different devices.

The major browsers run on one of three engines:

EngineMaintained byMain browsersOrigin
BlinkThe Chromium open-source projectChrome, Microsoft Edge and other browsers built on Chromium78Based on WebKit, announced in 20137
WebKitApple and the WebKit open-source projectSafari, and browsers on iPhone and iPad9Based on KDE’s KHTML, first shipped in Safari in 200310
GeckoMozillaFirefox6Mozilla’s NGLayout engine of 199811

On iPhone and iPad, Apple’s App Store rules say apps that browse the web must use Apple’s WebKit framework and WebKit JavaScript. Developers may apply for an entitlement to use another engine in the European Union and Japan.9 Outside those exceptions, a browser branded Chrome or Firefox on an iPhone still draws pages with WebKit.

The family tree, 1990 to 2021

The first browsers at CERN

The first browser was written by Tim Berners-Lee at CERN. He began work on a graphical browser and editor for the NeXT computer in October 1990 and called it WorldWideWeb.12 It could browse web, news, FTP and local files, but it could edit only local files, because the HTTP method for saving a page back to a server, PUT, was not yet implemented. It was later renamed Nexus, to avoid confusion between the program and the World Wide Web itself.13 By the end of 1990 the first web server and browser were running at CERN.14

Few people had a NeXT computer, so work started on a simpler line-mode browser, which showed plain text and could run on any system. It was written by Nicola Pellow during her student work placement at CERN.14 The W3C’s history of the project says she joined in November 1990 and that the line-mode browser was released to a limited audience in March 1991, on VAX, RS6000 and Sun4 computers.12 CERN’s history page says Berners-Lee’s 1991 software release included the line-mode browser, server software and a library for developers.14

Sources differ on when it became public. CERN’s 2013 page about recreating the browser says it was “launched in 1992,” and calls it the first readily accessible browser and the first with a cross-platform codebase.15

Other teams soon wrote graphical browsers for computers running the X Window System: Erwise from Finland in April 1992, Pei Wei’s Viola in May 1992 and Tony Johnson’s Midas at SLAC by January 1993.12

Mosaic, Netscape and Internet Explorer

At the National Center for Supercomputing Applications (NCSA) at the University of Illinois, developer Eric Bina and student Marc Andreessen built NCSA Mosaic.16 NCSA released a first alpha of Mosaic for X in February 1993, and Andreessen announced version 1.0 on April 21, 1993.1217 NCSA’s project page quotes a description of it as the first published browser that automatically displayed pictures along with text.16 By September 1993 there were working versions for X, Windows and the Macintosh.12

Netscape Communications of Mountain View, California, shipped Netscape Navigator 1.0 on December 15, 1994, for Windows, Macintosh and X Window System computers. It was free for evaluation, academic and non-profit use, and Netscape set the single-user price for commercial use at $39.18

Microsoft did not write its first browser from scratch. It licensed Spyglass Mosaic from the company Spyglass under an agreement dated December 12, 1994, and in August 1995 began distributing Internet Explorer, which Spyglass’s annual report calls “a WWW browser based on Spyglass Mosaic,” with Windows 95.19 Internet Explorer’s engine is known as Trident, or MSHTML.20

The race between Netscape and Microsoft that followed, and the later contest among Firefox, Safari and Chrome, are covered in the browser wars entry. The rest of this section follows the engines.

Gecko

In 1998 the Mozilla project was building a new layout engine called NGLayout, earlier code-named Raptor. A mozilla.org note dated December 10, 1998 says Netscape had started using the name Gecko for NGLayout and a related project, XPFE, while mozilla.org would keep calling the engine NGLayout.11 Gecko is now the engine of Mozilla’s Firefox.6

From KHTML to WebKit to Blink

On January 7, 2003, Apple released a free public beta of Safari, with a rendering engine “based on KHTML, from KDE’s Konqueror open source project.” Apple said it would contribute its improvements back to the open-source community.10 On June 7, 2005, Apple opened the WebKit source code repository and bug database to the public. The code included WebCore and JavaScriptCore, Apple’s forks of KDE’s KHTML and KJS.4

Google’s Chrome arrived in September 2008. Announcing it on September 1, Google said the beta would launch the next day in more than 100 countries, that it used components from Apple’s WebKit and Mozilla’s Firefox, and that each tab ran in an isolated “sandbox” so one tab could not crash another.21

On April 3, 2013, Google announced Blink, “a new open source rendering engine based on WebKit,” for the Chromium project. Software engineer Adam Barth wrote that Chromium used a different multi-process architecture from other WebKit-based browsers, and that supporting several architectures had made both projects more complex and slowed them down.7 Two months earlier, on February 13, 2013, Opera Software had said its browser would move to WebKit and Chromium rather than keep developing its own rendering engine.22

From EdgeHTML to Chromium Edge

Microsoft made its own break in 2015. In a February 26, 2015 post, Charles Morris of the Project Spartan team wrote that Microsoft’s new engine “began as a fork of MSHTML.dll.” The fork let the team remove document modes (settings that made Internet Explorer imitate its older versions) and other legacy behaviors, while the old engine stayed largely unchanged for businesses that depended on it.23 The new browser, Microsoft Edge, arrived with the launch of Windows 10 in July 2015. Its engine, EdgeHTML, was at version 12 for that launch, and Microsoft released EdgeHTML 13 that November.24

On December 6, 2018, Microsoft said it intended to adopt the Chromium open-source project for Edge on the desktop and to become a significant contributor to Chromium.25 The Chromium-based Edge came out of preview on January 15, 2020, for all supported versions of Windows and macOS, in more than 90 languages.8 Support for the old version, by then called Microsoft Edge Legacy, ended on March 9, 2021.26 Edge still carries Internet Explorer’s engine for one job: its IE mode uses the Chromium engine for modern sites and the Trident MSHTML engine from Internet Explorer 11 for old sites that an organization lists.20

Market share in 2026

StatCounter Global Stats publishes monthly browser shares measured in page views. As of September 2026, Chrome had 66.36% of page views worldwide across all devices, followed by Safari at 18.27% and Edge at 6.04%.27 The order changes by device and by country:

BrowserWorldwide, all devices27Worldwide, desktop28Worldwide, mobile29United States, all devices30
Chrome66.36%65.09%67.64%53.82%
Safari18.27%8.06%24.71%30.42%
Edge6.04%14.64%Not in top six7.51%
Firefox2.80%6.11%0.69%3.33%
Samsung Internet2.31%Not in top six3.50%1.80%
Opera1.77%2.38%1.38%1.18%

Edge’s share on desktop computers, 14.64%, is more than twice its share across all devices. Of the four views in the table, Safari’s share is highest in the United States, at 30.42% of page views. StatCounter’s desktop list also includes Brave at 1.12%, and its mobile list UC Browser at 0.78%.283029

These are shares by browser brand, not by engine. Because of Apple’s rules, page views from Chrome or Firefox on an iPhone are still drawn by WebKit, so engine shares cannot be read straight off the brand figures.9

StatCounter’s tracking code is installed on more than 1 million sites, and its global figures rest on more than 3 billion page views a month. It counts page views, not unique visitors, and says it works to remove bots. It also notes that the newer browsers Comet, Dia and ChatGPT Atlas identify themselves as Chrome, so it records their use under Chrome.31

Evergreen browsers and automatic updates

An evergreen browser updates itself, so most people run a recent version without doing anything. Microsoft used the word for its own engine in 2016, calling EdgeHTML “our new evergreen engine.”24

Chrome normally installs updates in the background and applies them when the browser is closed and reopened. Its About Google Chrome page checks for updates and offers a Relaunch button when one is waiting.32 In March 2026 Google announced that Chrome would move from a four-week to a two-week release cycle, beginning with the stable release of Chrome 153 on September 8, 2026. An Extended Stable option for businesses stays on an eight-week cycle.33

Microsoft Edge follows Chromium: its release schedule says a new Beta or Stable major version is triggered by an equivalent Chromium release. Starting with version 152, Edge moved to a two-week major release cycle, with an eight-week Extended Stable option for managed business environments. Edge 154 reached the Stable channel on September 24, 2026.34 Firefox keeps a similar pace: Mozilla’s release calendar lists Firefox 158 as released on October 13, 2026 and Firefox 159 as due on October 27, 2026.35

Safari works differently. Apple includes Safari with the operating system, and a newer Safari comes from updating or upgrading macOS, iOS, iPadOS or visionOS.36 A device that can no longer install new operating system versions therefore stays on an older Safari.

Fast updates shorten the wait for new features, but a feature still has to ship in every engine before a site can count on it. Baseline, a support label published on Google’s web.dev site, calls a feature “newly available” once all the core browsers support it, and “widely available” 30 months after that date.37

Testing a site across browsers

Because the major browsers share three engines, testing by engine covers most visitors with a handful of checks. Baseline’s core browser set is Chrome on desktop and Android, Edge, Firefox on desktop and Android, and Safari on macOS and iOS.37

  • Automated tests. The testing tool Playwright can run the same tests on Chromium, WebKit and Firefox. Its WebKit build comes from WebKit’s latest main branch, so it can include changes that have not yet reached Safari, and it does not work with the branded Safari app.38
  • Phone emulation. Chrome’s DevTools device mode imitates phone screens on a computer, but Google calls it “a first-order approximation” of a mobile device and advises running the page on a real phone when in doubt, using remote debugging.39
  • Real iPhones and iPads. Pages open in Safari on iOS and iPadOS can be inspected from a Mac. Turn on Web Inspector under Settings, Apps, Safari, Advanced, connect the device by cable and pick the page from the Develop menu in Safari on the Mac. iOS simulators installed with Xcode can be inspected the same way.40

Timeline

  1. Chrome 153 starts Chrome's two-week release cycle33
  2. Support ends for Microsoft Edge Legacy, the EdgeHTML version26
  3. The Chromium-based Microsoft Edge is released for Windows and macOS8
  4. Microsoft says it intends to adopt the Chromium open-source project for Edge25
  5. Microsoft Edge arrives with Windows 10, on the EdgeHTML engine24
  6. Google announces Blink, a rendering engine based on WebKit7
  7. Google Chrome beta launches, using WebKit21
  8. Apple opens the WebKit source repository and bug database4
  9. Apple releases the Safari public beta, built on KHTML10
  10. A mozilla.org note records Netscape calling NGLayout "Gecko"11
  11. Microsoft begins distributing Internet Explorer, based on Spyglass Mosaic19
  12. Netscape Navigator 1.0 ships18
  13. Marc Andreessen announces NCSA Mosaic for X 1.017
  14. The line-mode browser is released to a limited audience at CERN12
  15. Tim Berners-Lee begins the WorldWideWeb browser and editor on a NeXT12

What it means for your website

You do not need to test every browser ever made. You need the engines your visitors use, on the devices they hold.

  • Test three engines, not one brand. Check each new page in Chrome or Edge (Blink), Safari (WebKit) and Firefox (Gecko), the engines behind Baseline’s core browsers.37 A page that works in Chrome on your laptop has not yet been tested in WebKit.
  • Use a real iPhone. In the United States, Safari had 30.42% of page views in September 2026, and iPhone browsers use WebKit outside Apple’s exceptions.309 Try your forms, menus, booking widget and checkout on the phone itself, since desktop emulation is only an approximation.39
  • Check your own numbers. Your site’s analytics show which browsers and devices your visitors use. Test those first, whatever the global figures say.
  • Look up newer features first. Before you or your designer rely on a new CSS or JavaScript feature, check whether it is Baseline “widely available.”37
  • Retest after changes on your side. Chrome and Edge now ship a major version every two weeks, so you cannot test each one.3334 Recheck key pages after you change the theme, add a plugin or embed a third-party tool, and try big changes on a staging site first.
  • On a hosted builder, such as Squarespace or Wix, the platform maintains its own templates. The custom code and embeds you add are yours to test.

Our guide to how a website works follows a page from the server to the screen, and the glossary explains responsive design, the approach that lets one page fit both phones and desktops.

See also

References

  1. Browser (glossary). MDN Web Docs, Mozilla. accessed October 22, 2026.
  2. Populating the page: how browsers work. MDN Web Docs, Mozilla. accessed October 22, 2026.
  3. V8 JavaScript engine. V8 project, Google. accessed October 22, 2026.
  4. Apple Opens WebKit CVS and Bug Database. KDE.news (dot.kde.org). 2005-06-07. accessed October 22, 2026.
  5. Inside look at modern web browser (part 1) (Mariko Kosaka). Chrome for Developers, Google. 2018-09-05. accessed October 22, 2026.
  6. Gecko (glossary). MDN Web Docs, Mozilla. accessed October 22, 2026.
  7. Blink: A rendering engine for the Chromium project (Adam Barth). Chromium Blog, Google. 2013-04-03. accessed October 22, 2026.
  8. New year, new browser: The new Microsoft Edge is out of preview and now available for download. Microsoft Windows Experience Blog. 2020-01-15. accessed October 22, 2026.
  9. App Review Guidelines, guideline 2.5.6. Apple Developer. accessed October 22, 2026.
  10. Apple Unveils Safari. Apple Newsroom. 2003-01-07. accessed October 22, 2026.
  11. Gecko (mozilla.org note on the NGLayout name). Mozilla (www-archive.mozilla.org). 1998-12-10. accessed October 22, 2026.
  12. A Little History of the World Wide Web (created by Robert Cailliau, maintained by Dan Connolly). W3C. accessed October 22, 2026.
  13. The WorldWideWeb browser. Tim Berners-Lee, W3C. accessed October 22, 2026.
  14. A short history of the Web. CERN. accessed October 22, 2026.
  15. Line Mode Browser 2013. CERN. accessed October 22, 2026.
  16. NCSA Mosaic. National Center for Supercomputing Applications, University of Illinois. accessed October 22, 2026.
  17. NCSA Mosaic 1.0 released (Marc Andreessen, www-talk mailing list). www-talk mailing list archive, University of Calgary. 1993-04-21. accessed October 22, 2026.
  18. Netscape Communications ships release 1.0 of Netscape Navigator and Netsite servers (press release posted to www-talk). www-talk mailing list archive, University of Calgary. 1994-12-15. accessed October 22, 2026.
  19. Spyglass, Inc., Form 10-K for the fiscal year ended September 30, 1996. U.S. Securities and Exchange Commission (EDGAR). 1996-12. accessed October 22, 2026.
  20. What is Internet Explorer (IE) mode?. Microsoft Learn. accessed October 22, 2026.
  21. A fresh take on the browser (Sundar Pichai and Linus Upson). Official Google Blog. 2008-09-01. accessed October 22, 2026.
  22. Opera gears up at 300 million users. Opera Software. 2013-02-13. accessed October 22, 2026.
  23. A break from the past: the birth of Microsoft's new web rendering engine (Charles Morris). Microsoft Edge Blog. 2015-02-26. accessed October 22, 2026.
  24. Microsoft Edge: 2015 in review (Charles Morris). Microsoft Edge Blog. 2016-01-21. accessed October 22, 2026.
  25. Microsoft Edge: Making the web better through more open source collaboration. Microsoft Windows Experience Blog. 2018-12-06. accessed October 22, 2026.
  26. End of support reminder for Microsoft Edge Legacy. Microsoft Learn (Lifecycle announcements). 2021-02-05. accessed October 22, 2026.
  27. Browser Market Share Worldwide (September 2026). StatCounter Global Stats. 2026-09. accessed October 22, 2026.
  28. Desktop Browser Market Share Worldwide (September 2026). StatCounter Global Stats. 2026-09. accessed October 22, 2026.
  29. Mobile Browser Market Share Worldwide (September 2026). StatCounter Global Stats. 2026-09. accessed October 22, 2026.
  30. Browser Market Share United States of America (September 2026). StatCounter Global Stats. 2026-09. accessed October 22, 2026.
  31. FAQ. StatCounter Global Stats. accessed October 22, 2026.
  32. Update Google Chrome. Google Chrome Help. accessed October 22, 2026.
  33. Get features faster with Chrome's two-week release cycle (Ben Mason and Deepak Ravichandran). Chrome for Developers, Google. 2026-03-03. accessed October 22, 2026.
  34. Microsoft Edge release schedule. Microsoft Learn. accessed October 22, 2026.
  35. Firefox release calendar. Mozilla (whattrainisitnow.com). accessed October 22, 2026.
  36. Update to the latest version of Safari. Apple Support. accessed October 22, 2026.
  37. Baseline. web.dev, Google. accessed October 22, 2026.
  38. Browsers. Playwright documentation. accessed October 22, 2026.
  39. Simulate mobile devices with device mode. Chrome DevTools documentation, Google. accessed October 22, 2026.
  40. Inspecting iOS and iPadOS. Safari Developer Tools documentation, Apple Developer. accessed October 22, 2026.

Web Building Encyclopedia. First published October 22, 2026. Last reviewed October 22, 2026. Every fact on this page is linked to a source we opened. Spotted an error? Write to [email protected].