How HTML, CSS and JavaScript work together in web pages

The three main technologies used to create modern web pages (HTML, CSS and JavaScript) each do different jobs.

HTML should be used only for structuring content.

Cascading Style Sheets should be used for applying all visual styles.

JavaScript should be used for (almost) all interactive functionality, and should always be referenced in separate files, never written into HTML.

How HTML, CSS and JS work

The web page you see in your browser may be a combination of structure, style and interactivity. These jobs are undertaken by 3 different technologies, which your browser knows how to interpret.

A web page may combine structure from HTML, style from CSS and functionality from JavaScript
HTML
marks the content up into different structural types, like paragraphs, blocks, lists, images, tables, forms, comments etc.
CSS
tells the browser how each type of element should be displayed, which may vary for different media (like screen, print or handheld device)
JavaScript
tells the browser how to change the web page in response to events that happen (like clicking on something, or changing the value in a form input)

The 2 most important things to learn about web page production are:

  1. Use the correct HTML tags that mean what they say.
  2. Keep your HTML, CSS and JavaScript code separate.

1. Use HTML properly

Different HTML tags describe the structure of content. Each one has its own meaning, and you should only ever use each one in accordance with its meaning, and in its proper place.

When people talk about "semantic" HTML, this is what they mean.

In the HTML section, I'll explain what each tag means, and where it's OK to use it.

2. Keep 'em separate

This simply means: Don't put JavaScript or CSS in your HTML. Put them in separate files, called in by your HTML pages.

There are lots of good reasons for this, the main ones being:

  • It's more logical and simpler. And simple is good.
  • Easier management: Keeping all your styles and functional code in one place makes it quicker, easier and safer to change in future.
  • Easier re-purposing: It's easier to design your web site to look or work differently on different user agents (e.g. handheld browsers, audio browsers, or TV)
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