Hvad er CSS?

CSS står for Cascading Style Sheets og er det primære sprog der bruges til at beskrive udseende og formattering på sider på Internettet samt markup dokumenter (fx HTML og XML) generelt.

A markup language like HTML was initially designed to provide information about formatting and looks itself, but it soon became clear that it would make much more sense to split this into two layers: Document Content and Document Presentation, with CSS fulfilling the task of the latter. Historically that is why HTML has tags like font, which sole purpose is to adjust font family, color and size locally, a job that is today handled by CSS. This allows the developer to re-use formatting rules across several places in the same document and even across multiple documents. Here's an example to prove my point, and don't worry if it's not entirely clear to you what it does - all aspects will be explained throughout this tutorial:

Gammeldags tekst formattering, hvor kun HTML bruges:

This is a piece of
<font face="Tahoma,Verdana,Arial" color="Blue" size="3"><i><b>text</b></i></font> with
<font face="Tahoma,Verdana,Arial" color="Blue" size="3"><i><b>highlighted</b></i></font> elements in
<font face="Tahoma,Verdana,Arial" color="Blue" size="3"><i><b>it</b></i></font>.

A more modern approach with CSS:

<style type="text/css">
.highlight {
	color: Blue;
	font-style: italic;
	font-weight: bold;
	font-size: 120%;
	font-family: Tahoma, Verdana, Arial;
}
</style>

This is a piece of
<span class="highlight">text</span> with
<span class="highlight">highlighted</span> elements in
<span class="highlight">it</span>.

Notice how I simply re-use the same set of rules across several HTML tags. This is already an advantage when using it three times, like we do in the example, but it doesn't end there - put the CSS in an external stylesheet file (more on that later) and you can use the same rules across your ENTIRE website. And how about when you decide that highlighted text should be red instead of blue? With the first approach, you would have to manually edit the tags everywhere you used it - with CSS, just change the single ".highlight" rule!

Summary

CSS allows you to easily apply rules about formatting and layout to your HTML elements and then re-use those rules across multiple elements and even pages. In this introduction, we looked at some CSS code, but we did not talk about how it works and why it looks the way it does - this will be the subject for the next couple of chapters, where we start from scratch and explain it all in details.