What Is a Browser?

Browsers and search engines are often confused, because almost every browser comes with a default search engine in the address bar. They are different things. A search engine, such as Google or Bing, explores the web and ranks pages for a query. A browser displays the page you have chosen.

The core of a browser is its rendering engine, the part that, according to MDN, turns a page's HTML, CSS and other resources into a visual representation. There are only a few engines. Chrome is built on the open-source Chromium project, whose engine is Blink. Safari uses WebKit, and Firefox uses Gecko, the engine developed by Mozilla. That is why the same page can look slightly different from one browser to another.

When someone types an address or clicks a link, the browser requests the page from the server. From the HTML it builds the page structure, the DOM, and downloads what the code links to: stylesheets, scripts, images and fonts. It then runs the JavaScript and paints the result. If a page builds its content with JavaScript in the browser, what you see depends on that code running without errors.

The browser also stores data on the device. Its cache keeps resources it has already downloaded so it does not have to request them again, and cookies hold data that sites ask it to remember. The major browsers also have a back/forward cache, the bfcache. According to Google, Chrome has had it since version 96, as have Firefox and Safari, and it makes the back and forward buttons near instant.

Google sees pages much as a browser does. According to its documentation, Google Search runs JavaScript with an evergreen version of Chromium. What works in a current version of Chrome is therefore likely to work for Googlebot too, although Google warns that Googlebot caches aggressively and may use outdated JavaScript or CSS files.

The browser is also the source of field data on page experience. CrUX, Google's field dataset for the Core Web Vitals, is collected from real Chrome users on desktop and Android who have enabled certain settings. Chrome on iOS does not contribute data, and neither does Safari. In Spain this matters: according to StatCounter, which measures visits on the sites in its network, Chrome accounted for 74% of visits in September 2026, Safari for 15.4%, Edge for 3.3%, Samsung Internet for 2.7%, Firefox for 2.2% and Opera for 1.5%, in rounded figures.

On 14 October 2026 we checked whether a glossary page on zds.es reaches Chrome, Safari and a request identifying itself as Googlebot in the same form. All three received the same HTML, 190,480 bytes each time. The only difference is an email address that Cloudflare encodes afresh on every request.

• Test pages in Chrome and in Safari, on desktop and on mobile.
• Use the URL Inspection tool in Search Console to see how Google renders a page.
• Serve the same content to every browser and crawler.
• Include the viewport meta tag so that mobile browsers fit the page to the screen.
• Rename JavaScript and CSS files when their content changes, so that neither the browser nor Googlebot uses old versions.
• Keep in mind that CrUX data covers only part of Chrome's users.

• Confusing a browser with a search engine.
• Testing only in desktop Chrome, when a large share of visits comes from Safari and from mobile devices.
• Assuming that Googlebot runs every piece of JavaScript exactly as the developer's browser does.
• Serving different content depending on the browser or crawler that requests the page.
• Reading CrUX field data as if it included Safari users.