system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The top used Internet Browsers and how they're different.
gpt
As you may know, internet pages are written with HTML and various other languages. A nice analogy is to consider a web page as a museum. Let me explain.

The HTML is the architecture of the building itself. It's the building blocks of the page that tell you what things are and what they do. Using HTML, you give the browser data about what the page actually contains.

CSS is the paintings, or galleries, or whatever makes the museum appealing. CSS makes web pages look good. Without CSS, most if not all modern websites would look absolutely horrible. Just like a museum would just be a big empty building without its exhibits.

JavaScript is the museum curators. They let the guests experience the museum, and customize their experience. Similarly, all the interactive elements of websites (show/hide buttons, upvotes, etc) are JavaScript. 

PHP is the workers of the museum. They keep everything going behind the scenes. PHP runs on the server and updates things like databases, just as a worker would change the museum (put in new exhibits, build expansions, etc).

Now, how does this relate to browsers? Well, a web page is just a file full of code. It's the browser's job to display this to you, the end user. In the early days of the internet, there wasn't much to display. Especially before CSS, most pages looked the same no matter how they were displayed and fairly bland. However, now, there is new technology; CSS3 has nice gradients, for example, and HTML5 has video and audio elements (you have probably seen Youtube's new HTML5 player at some point). The browsers have to update in order to work with these new forms of old technology, and they don't all do it the same. Most browsers don't even support all of it, and there's a website (I can't remember it now) which will tell you what browsers support what facets of HTML5 and CSS3.

TL;DR: They all display pages differently and support different web technology.