min-height anything over. max-height anything under. More on reddit.com

Since this is Explain like I'm five: What javascript is As you said, HTML is for content and CSS is for styling. Javascript, however, is a programming language. When used in the browser, it's mostly used to make things fancy and add functionality that would otherwise require a full reload from the page. (Javascript is not used exclusively on the browser, it's a programming language that can do everything any other language like python or C++ can, and in fact, although it has many flaws, it's a pretty cool language for web programming). How javascript is used So where's javascript used on reddit? If reddit existed in 1995, before javascript came along, clicking the save button to post this comment would send a POST HTTP request to reddit and the page would reload. Even if I could send the POST request without reloading somehow, my version of the webpage would not change (the comment would not appear), since there would be no way to update the HTML of the page dynamically while the user has the page open. There would be no other way to get content from the server. However, now that javascript exists, clicking save to post will not submit a request directly. Instead, it will trigger some javascript code that is executed within the browser. This piece of javascript can do pretty much everything with the data. In this case, it will check that the comment textbox is not empty, if it's not it will take it's content and send it to the server without reloading the page. The server (and I will explain why/how later) will receive the request, process it in some way, save it in some kind of database and return a reply that will hopefully be along the lines of "Ok, I saved the comment, kkthxbb". The javascript client will receive this reply and update your version of the webpage by closing the textbox and making the comment visible. What else is needed Of course HTML/CSS/JS is not all that's required in making a website. What's this server thing? Well, if you're going to have persistent content and many users in your application, you need to somehow save data somewhere. In reddit's case, links, posts, comments and up/down votes. That happens in a database, a program responsible for keeping data and performing operations on the data (updates, lookups, deletions) fast, really fast. The rest of the application interacts with this database, performing operations like "Add an new comment to the comments table and keep a reference to the containing post". Clearly, the users should be able to somehow access this database. I need some kind of access to reddit's database to post this comment. Why javascript isn't enough So why can't client-side javascript handle that? It can! Theoretically you could have a dynamic app like reddit that runs entirely on the browser. Database operations and data validation (ensuring my comment is not a a quazillion bytes long and doesn't contain any harmful code) would be done entirely on the user's browser. But that wouldn't be a very clever thing to do, since javascript code is visible to the user (there's no way to prevent this effectively since it's executed on the user's computer). On chrome, pressing ctrl+alt+I (if I remember correctly) let's you see all of reddit's javascript, and run your own javascript commands. That means it would be redicioulously easy to bypass any kind of validation and post whatever I want on the comments, even harmful code. Even worse, since the client would have read/write access to the database anybody would be able to delete and update anything. Clearly we don't want that. Whatever kind of app you're building, you'll almost always want to limit database access and validate user data in a safe way, away from the client. That's why there's a server application running on reddit's servers. That application has complete read/write access to the database and performs data validation, however users can't interact with it directly. You can only send requests like "POST a new comment" with arguments (the comment itself in this case) to the server. The server code (for reddit it's written in python) will run in an entirely different computer, process and validate your request and send you an answer. You could say that the server-side code is a middleman between you and the database. It's important to realize that the choice of language has nothing to do with safety. Performing data validation with javascript is safe, as long as you're not doing it on the client side. Javascript also works on the server-side. It depends on where the code is run, not in what language it's written. Conclusion Additionally, a website may have all kinds of code running on the backend. Reddit has an algorithm that chooses what posts will go to the front page and what posts will not. Usually when writing an web app, you use an existing database program like MySQL, CouchDB, Redis or MongoDB to store your data and perform operations on it. Then you have to write the backend code that respons to user requests, processes data and interacts with both the database and the clients, and the HTML and CSS to render data on the client and optionally some client-side javascript to add some logic on the client-side. TL;DR: You need back-end code for security reasons. More on reddit.com

HTML defines content, CSS decorates it and helps tell how it is displayed.

CSS stands for Cascading Stylesheet. This means that there is a hierarchy of style attributes overwriting other attributes that affect the same elements.

Think of it like this. Bob Ross does an oil painting and starts off with a canvas covered in liquid white. This is the lowest level on the hierarchy. It applies to the entire body of the painting, like a class applied to the body tag.

On that he paints some divs of class "mountain". The mountain class has the attribute paint-color:Van Dyke Brown. Since the div is within body the paint-color:Van Dyke Brown attribute overwrites the liquid white and you see the Van Dyke Brown over the liquid white.

He adds some divs of class "happy little tree" which are growing wherever they like, which happens to be on the mountains, so these happy little tree divs are within the mountain divs. Their paint-color:Sap Green is within the mountain div, so where the happy little tree divs are is painted with Sap Green on top of the Van Dyke Brown mountains.

Then Bob adds another happy little tree div. This one he wants to be Alizarin Crimson because its his world and it can be whatever fucking color he god damn pleases. Rather than giving it a special id and using the # selector Bob makes the happy little accident of using an inline style. This overwrites the happy little tree class's paint-color:Sap Green attribute with an inline paint-color:Alizarin Crimson because inline styles cascade over classes.

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Like you're five? Okay then! Imagine your house. You've got bedrooms, stuff in your bedrooms, people living in the house and lots of colours and materials everywhere. HTML5 is the main backbone of the house, it controls what objects exist inside the house. CSS is what makes the house look good. It controls where these objects go. It controls what colour they are, how big they are, and really just about anything to do with their appearance. JavaScript is a little more complicated. It's something that lets the house DO things. So say you had a white microwave sitting in your kitchen. The microwave is there because HTML told it to exist. It's white and sitting in your kitchen because that's how CSS told it to appear... And finally it's able to switch on and heat things because that's what JavaScript is doing. JavaScript works with the existing HTML and CSS code and manipulates them slightly to carry out certain actions when it needs to. Finally, the last main web language, PHP. This is sort of like the "daddy" of the house. PHP starts organising the house. It clumps groups of similar typed HTML code (like say all the bedrooms) into one command. That means that when this one command is announced, suddenly a bedroom will appear. So say the daddy decides that it no longer wants 3 bedrooms and 2 bathrooms in the house, but it wants 4 bedrooms and 4 bathrooms. Rather than going through the trouble of making it all again, he just calls out this one command. PHP, unlike the other languages is all maintained behind the scenes. What you'll see is the resulting bits and bobs of HTML and CSS that you can recognise, but really it's PHP that's controlling what to show you and what to keep hidden away for later use because it isn't currently necessary. More on reddit.com