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<h1 class="nav-title">How CSS works</h1>
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<a href="#What_is_CSS?" class="nav-link">What is CSS?</a>
<a href="#How_does_CSS_affect_HTML?" class="nav-link">How does CSS affect HTML?</a>
<a href="#How_does_CSS_actually_work?" class="nav-link">How does CSS actually work?</a>
<a href="#About_the_DOM" class="nav-link">About the DOM</a>
<a href="#How_to_apply_CSS_to_your_HTML" class="nav-link">How to apply CSS to your HTML</a>
<a href="#What's_next?" class="nav-link">What's next?</a>
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<section class="main-section" id="What_is_CSS?">
<header>
<h1>What is CSS?</h1>
</header>
<p>As we have mentioned before, CSS is a language for specifying how documents are presented to users — how they
are styled, laid out, etc.</p>
<p>A document is usually a text file structured using a markup language — HTML is the most common markup language,
but you will also come across other markup languages such as SVG or XML.</p>
<p>Presenting a document to a user means converting it into a usable form for your audience. Browsers, like Firefox,
Chrome or Internet Explorer, are designed to present documents visually, for example, on a computer screen,
projector or printer.</p>
</section>
<section class="main-section" id="How_does_CSS_affect_HTML?">
<header>
<h1>How does CSS affect HTML?</h1>
</header>
<p>Web browsers apply CSS rules to a document to affect how they are displayed. A CSS rule is formed from:</p>
<ul>
<li>A set of properties, which have values set to update how the HTML content is displayed, for example I want
my element's width to be 50% of its parent element, and its background to be red.</li>
<li>A selector, which selects the element(s) you want to apply the updated property values to. For example, I
want to apply my CSS rule to all the paragraphs in my HTML document.</li>
</ul>
<p>A set of CSS rules contained within a stylesheet determines how a webpage should look. You will learn a lot more
about what CSS syntax looks like in the next article of the module — CSS Syntax.</p>
<h2>A quick CSS example</h2>
<p>The above descriptions may or may not have made sense, so let's make sure things are clear by presenting a quick
example. First of all, let's take a simple HTML document, containing an <h1> and a <p> (notice
that a stylesheet is applied to the HTML using a <link> element):</p>
<pre>
<code>
<span><<span></span>!</span>DOCTYPE html<span>></span>
<span><</span>html<span>></span>
<span><</span>head<span>></span>
<span><</span>meta charset="utf-8"<span>></span>
<span><</span>title<span>></span>My CSS experiment<span><</span><span>/</span>title<span>></span>
<span><</span>link rel="stylesheet" href="style.css"<span>></span>
<span><</span><span>/</span>head<span>></span>
<span><</span>body<span>></span>
<span><</span>h1<span>></span>Hello World!<span><</span><span>/</span>h1<span>></span>
<span><</span>p<span>></span>This is my first CSS example<span><</span><span>/</span>p<span>></span>
<span><</span><span>/</span>body<span>></span>
<span><</span><span>/</span>html<span>></span>
</code>
</pre>
<p>Now let's look at a very simple CSS example containing two rules:</p>
<pre>
<code>
h1 {
color: blue;
background-color: yellow;
border: 1px solid black;
}
p {
color: red;
}
</code>
</pre>
<p>The first rule starts with an h1 selector, which means that it will apply its property values to the <h1>
element. It contains three properties and their values (each property/value pair is called a declaration):</p>
<ol>
<li>The first one sets the text color to blue.</li>
<li>The second sets the background color to yellow.</li>
<li>The third one puts a border around the header that is 1 pixel wide, solid (not dotted, or dashed, etc.),
and colored black.</li>
</ol>
<p>The second rule starts with a p selector, which means that it will apply its property values to the <p>
element. It contains one declaration, which sets the text color to red.</p>
<h2>Active learning: Writing your first CSS</h2>
<p>Now we'll give you a chance to write your own bit of CSS. You can do this using the Input areas below, in the
live editable example. In a similar fashion to what you saw above, you've got some simple HTML elements,
and some CSS properties. Try adding some simple declarations to your CSS, to style the HTML.</p>
</section>
<section class="main-section" id="How_does_CSS_actually_work?">
<header>
<h1>How does CSS actually work?</h1>
</header>
<p>When a browser displays a document, it must combine the document's content with its style information. It processes
the document in two stages:</p>
<ol>
<li>The browser converts HTML and CSS into the DOM (Document Object Model). The DOM represents the document in
the computer's memory. It combines the document's content with its style.</li>
<li>The browser displays the contents of the DOM.</li>
</ol>
</section>
<section class="main-section" id="About_the_DOM">
<header>
<h1>About the DOM</h1>
</header>
<p>
Rather than a long, boring explanation, let's take an example to see how the DOM and CSS work together.
</p>
<p>Let's assume the following HTML code:</p>
<pre>
<code>
<p>
Let's use:
<span>Cascading</span>
<span>Style</span>
<span>Sheets</span>
</p>
</code>
</pre>
<p>In the DOM, the node corresponding to our <p> element is a parent. Its children are a text node and the
nodes corresponding to our <span> elements. The SPAN nodes are also parents, with text nodes as their
children:
</p>
<pre>
<code>
P
├─ "Let's use:"
├─ SPAN
| └─ "Cascading"
├─ SPAN
| └─ "Style"
└─ SPAN
└─ "Sheets"
</code>
</pre>
<p>This is how a browser interprets the previous HTML snippet —it renders the above DOM tree and then outputs it
in the browser like so:</p>
<pre>Let's use: Cascading Style Sheets</pre>
<h2>Applying CSS to the DOM</h2>
<p>Let's say we added some CSS to our document, to style it. Again, the HTML is as follows:</p>
<pre>
<code>
<p>
Let's use:
<span>Cascading</span>
<span>Style</span>
<span>Sheets</span>
</p>
</code>
</pre>
<p>If we apply the following CSS to it:</p>
<pre>
<code>
span {
border: 1px solid black;
background-color: lime;
}
</code>
</pre>
<p>The browser will parse the HTML and create a DOM from it, then parse the CSS. Since the only rule available in
the CSS has a span selector, it will apply that rule to each one of the three spans.</p>
</section>
<section class="main-section" id="How_to_apply_CSS_to_your_HTML">
<header>
<h1>How to apply CSS to your HTML</h1>
</header>
<p>There are three different ways to apply CSS to an HTML document that you'll commonly come across, some more useful
than others. Here we'll briefly review each one.</p>
<h2>External stylesheet</h2>
<p>You've already seen external stylesheets in this article, but not by that name. An external stylesheet is when
you have your CSS written in a separate file with a .css extension, and you reference it from an HTML <link>
element. The HTML file looks something like this:</p>
<pre>
<code>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>My CSS experiment</title>
<link rel="stylesheet" href="style.css">
</head>
<body>
<h1>Hello World!</h1>
<p>This is my first CSS example</p>
</body>
</html>
</code>
</pre>
<p>And the CSS file:</p>
<pre>
<code>
h1 {
color: blue;
background-color: yellow;
border: 1px solid black;
}
p {
color: red;
}
</code>
</pre>
<p>This method is arguably the best, as you can use one stylesheet to style multiple documents, and would only need
to update the CSS in one place if changes were needed.</p>
<h2>Internal stylesheet</h2>
<p>An internal stylesheet is where you don't have an external CSS file, but instead place your CSS inside a <style>
element, contained inside the HTML head. So the HTML would look like this:</p>
<pre>
<code>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>My CSS experiment</title>
<style>
h1 {
color: blue;
background-color: yellow;
border: 1px solid black;
}
p {
color: red;
}
</style>
</head>
<body>
<h1>Hello World!</h1>
<p>This is my first CSS example</p>
</body>
</html>
</code>
</pre>
<p>This can be useful in some circumstances (maybe you're working with a content management system where you can't
modify the CSS files directly), but it isn't quite as efficient as external stylesheets — in a website, the
CSS would need to be repeated across every page, and updated in multiple places if changes were required.</p>
<h2>Inline styles</h2>
<p>Inline styles are CSS declarations that affect one element only, contained within a style attribute:</p>
<pre>
<code>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>My CSS experiment</title>
</head>
<body>
<h1 style="color: blue;background-color: yellow;border: 1px solid black;">Hello World!</h1>
<p style="color:red;">This is my first CSS example</p>
</body>
</html>
</code>
</pre>
<p>
Please don't do this, unless you really have to! It is really bad for maintenance (you might have to update the same information
multiple times per document), and it also mixes your presentational CSS information with your HTML structural
information, making the CSS harder to read and understand. Keeping your different types of code separated
and pure makes for a much easier job for all who work on the code.
</p>
<p>
The only time you might have to resort to using inline styles is when your working environment is really restrictive (perhaps
your CMS only allows you to edit the HTML body.)
</p>
</section>
<section class="main-section" id="What's_next?">
<header>
<h1>What's next?</h1>
</header>
<p>At this point you should understand the basics of how CSS works, and how browsers deal with it. Next, you'll
move on to learn about CSS syntax.</p>
</section>
<p id="credits" class="credits">* All the documentation in this page is taken from MDN</p>
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