HTTP ResponsesWe have learned that web servers respond to our requests by sending back various types of data.

HTML

One of the most important types of data returned by web servers is called HTML.

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HTML stands for HyperText Markup Language. We can think of HTML as the content of our webpage, plus some extra information which tells the browser about the content.

For example, it may tell the browser how the content is structured, or how to get from the current web page to another web page, or how the page should be displayed. In the next few lessons we’ll start learning how to write our own HTML. For now the important lesson is:

HTML is the content of our webpage formatted in a way that web browsers can understand

CSS

CSS is the next most important type of data returned by a HTTP Response.

CSS stands for Cascading Style Sheets. This is a special display language which tells the web browser how to present the content in the HTML. This is what makes websites look pretty (or ugly!).

Again, we will learn how to write our own CSS later in this course, but for now the important lesson is:

CSS tells web browsers how to display the HTML content

HTML without CSS

It is possible to have HTML without any CSS. Web browsers are perfectly happy to display just HTML, or even just plain text.

HTML also contains several ways to display text in different ways, e.g. font size, color, bold, italic, even very simple animations. However, in modern web development it is very bad practice to change your display using HTML.

This is because CSS has many advantages over HTML display:

1) CSS is much more powerful than HTML for styling

2) CSS can be re-used across many pages on your site

3) It is much easier to change your site if all the display logic is in one central place: your CSS files

It can be very very difficult to make changes to a site when some display logic is in HTML and some is in CSS, so it is considered good practice to keep your display logic purely in CSS, and keep HTML purely for content.

This is part of the software development practice known as separation of concerns – making sure that each part of your web site is responsible for only what it should be responsible for.

HTML: Content

CSS: Display

JavaScript

JavaScript is another important type of data returned by responses.

JavaScript is a coding language, unlike HTML and CSS which are markup languages. In other words, JavaScript is much more like a standard programming language.

JavaScript is the coding language that tells websites how to behave. It is responsible for the functionality of a modern website, including loading data in the background, changing CSS dynamically to provide animations, validating data and handling user interactions.

JavaScript code runs locally in your browser, not on the web server. The server sends the JavaScript code down as part of the response, and your browser then runs it on your computer. This allows your computer to understand how to interact with the website.

We will learn more about JavaScript later in the course too. For now, the important lesson is:

JavaScript is responsible for the behaviour of a web page

HTML: Content – a markup language

CSS: Display – a markup language

JavaScript: Behaviour – a programming language

As we progress we will understand more how each of these types of response is generated, and how the browser knows how to request all of this data from the web server.


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