Why CSS exists

As the Web grew in popularity in the 90s, the intent to apply a specific design to a website grew as well. Web developers relied upon specific HTML tags to enhance webpages:

<basefont> defined a font for a whole HTML document
<font> defined a typeface, a color, and a size for the text it contains
<center> horizontally centered all its content
<big> increased the size of the text
<strike> rendered text with a strikethrough

Several HTML attributes could also be used:

bgcolor defined a background color on the element
text defined the text color
several margin attributes could be used to add space on any side of the element

Why avoid tables

But most of all, in order to create columns, visually align elements, and generally position elements relatively to each other, Web developers used the <table> element to design their webpages because it naturally provided a visual grid:

<table>
  <thead>
    <tr>
      <th>Logo</th>
      <th colspan="2">Tagline</th>
    </tr>
  </thead>
  <tfoot>
    <tr>
      <th colspan="3">Copyright 2015</th>
    </tr>
  </tfoot>
  <tbody>
    <tr>
      <td>Left menu</td>
      <td>Main content</td>
      <td>Right sidebar</td>
    </tr>
  </tbody>
</table>

This approach was cumbersome for several reasons:

HTML tables are verbose: they require a lot of boilerplate code
the markup was semantically wrong: tables should be used for multidimensional data
changing the layout required to change the markup: if we wanted to move the left column to the right, we had to modify the HTML structure
tables were prone to syntax errors: rows and cells need to be ordered and nested in a specific way to be valid
the markup was unreadable: tables were nested within tables to provide additional columns within columns

That is why using tables for layout purposes was slowly abandoned, and CSS was used instead.

What CSS is

CSS stands for Cascading Style Sheets. Its purpose is to style markup languages (like HTML or XML). Therefore, CSS is worthless on its own, unless associated with an HTML document.

CSS brings an HTML document to life, by choosing fonts, applying colors, defining margins, positioning elements, animating interactions, and much more.

How CSS works

How CSS works is by selecting an HTML element (like a paragraph), choosing a property to alter (like the color), and applying a certain value (like red):

p{ color: red;}

The word “Style” can be deceiving. You might think CSS is only used to change the text’s color, size, and font. But CSS is able to define an HTML document’s layout, by defining heights, widths, inner and outer margins, positions, columns…

Where do I write CSS?

CSS as an attribute

You can write CSS directly on an HTML elements, by using the style attribute:

<p style="color: red;">This text is important.</p>

CSS in the <head>

You can use a <style> tag in the <head> of your HTML document:

<html>
  <head>
    <title>Hello World</title>
    <style type="text/css">
      p{ color: red;}
    </style>
  </head>
  <body>
    <p>This paragraph will be red.</p>
  </body>
</html>

CSS in a separate file

You can write your CSS in a separate file with a .css extension, and then link it to your HTML by using the <link> HTML tag.

p{ color: red;}

<html>
  <head>
    <title>Hello World</title>
    <link rel="stylesheet" type="text/css" href="style.css">
  </head>
  <body>
    <p>This paragraph will be red.</p>
  </body>
</html>

It is the HTML document who “calls” the CSS file, in this case a file called style.css located in the same folder as the HTML file.

This 3rd method of using a separate CSS file is preferred.

Why not style directly in the HTML?

Because we want to separate the content (HTML) from its presentation (CSS).

If you want to visualize the purpose of this distinction, head towards the wonderful CSS Zen Garden: each design uses the exact same HTML but a different CSS each time.

It makes maintenance easier as well: the same CSS file can be used for a whole website. It provides flexiblity: focus on the content on one side, the styling on the other. SEO purposes, different concerns.

4.2 Why CSS exists

Separating content and styling