A web browser is the program you use to visit websites. Google Chrome, Apple Safari, Microsoft Edge and Mozilla Firefox are the best known. You give it an address, it fetches the page from a server, and it turns the page's code into the text, images and buttons you see and click.
What a browser does after you press Enter
- It reads the URL and looks up the domain's IP address through DNS.
- It opens a secure connection to the web server and requests the page, sending along any cookies for that site and a user agent string describing itself.
- It parses the returned HTML, then fetches the CSS, JavaScript, fonts and images it references.
- Its rendering engine works out the layout and paints the page; its JavaScript engine runs the scripts that make it interactive.
Files it has already downloaded can be reused from the browser cache, which is why a second visit is usually faster.
Engines: why there are really only three
Under the brand names sit a few rendering engines. Chrome, Edge, Opera, Brave and Samsung Internet all use Blink. Safari uses WebKit, and on iPhone and iPad every browser has historically been required to use WebKit too, although Apple now allows other engines in the EU. Firefox uses Gecko. So testing a site in Chrome, Safari and Firefox covers almost every visitor.
A date picker that fails on iPhone
A salon owner's new booking page looks perfect on her laptop in Chrome, but a client says the date picker does nothing on her iPhone. The cause is a feature Safari handles differently. This is the most common browser problem small sites face: the site was only checked in one engine, and iPhone users, who all browse through WebKit in most countries, hit the gap.
When browsers matter for your website
You rarely need to think about which browser people use, because modern browsers follow the same web standards closely. It matters when you use a very new CSS or JavaScript feature, when a third-party widget misbehaves, or when privacy features (like Safari's tracking prevention) change how your analytics or ad cookies behave.
Browser-related mistakes
- Testing only in your own browser. Check Safari on an iPhone and Chrome on Android at minimum.
- Telling customers to "clear their cache" instead of versioning your files so updates reach everyone.
- Sniffing the user agent to block browsers. Detect features, not brands.
- Assuming everyone has the latest version. Older phones may run outdated browsers that miss new features.
Related terms
- User agent: how a browser identifies itself to servers.
- Browser cache: the browser's local copy of files.
- Cookie: small data a browser stores for sites.
- Web server: the other side of every browser request.