Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS describes how elements should be rendered on screen, on paper, in speech, or on other media.

CSS is among the core languages of the open web and is standardized across Web browsers according to W3C specifications. Previusly, development of various parts of CSS specification was done synchronously, which allowed versioning of the latest recommendations. You might have heard about CSS1, CSS2.1, CSS3. However, CSS4 has never become an official version.

From CSS3, the scope of the specification incresed significantly and the progress on different CSS modules started to differ so much, that it became more effective to develop and release recommendations separately per module. Instead of versioning CSS specification, W3C now periodically takes a snapshot of the latest stable state of the CSS specification.

This guide assumes you have the following basic background:

• A general understanding of the internet and the World Wide Web (WWW).
• Good working knowledge of Hyper Text Markup Language (HTML).

Like HTML, CSS is not a programming language. It's not a markup language either. CSS is a style sheet language CSS is what you use to selectively style HTML elements. For example, this CSS selects paragraph text, setting the color to red:

p {
    color: red;
}

Let's try it out! Using a text editor, paste the three lines of CSS (above) into a new file. Save the file as style.css in a directory named styles.

To make the code work, we still need to apply this CSS (above) to your HTML document. Otherwise, the styling won't change the appearance of the HTML. (If you haven´t been following our project, pause here to read Dealing with files and HTML basics).

1. Open your index.html file. Paste the following line in the head: <link href="styles/style.css" rel="stylesheet">
2. Save index.html and load it in your browser. You should see something like this:

If your paragraph text is red, congratulations! Your CSS is working.

Let's dissect the CSS code for red paragraph text to understand how it works:

The whole structure is called a ruleset (the term ruleset is often referred to as just rule). Note the names of the individual parts:

Selector

this is the HTML element name at the start of the ruleset. It defines the element(s) to be styled (in this example), <p> elements). To style a different element, change the selector.

Declaration

This is a single rule like color: red;. It specifies which of the element's properties you want to style.

Properties

These are ways in which you can style an HTML element. (In this example, color is a property of the <p> elements). In CSS you choose which properties you want to affect in the rule.

Property value

To the right of the property, after the colon, there is the porperty value. This chooses one out of many possible appearances for a given porperty. (For example, there are many color values in addition to red).

Note the other important parts of the syntax:

• Apart from the selector, each ruleset must be wrappped in curly braces. ({})
• Within each declaration, you must use a colon (:) to separate the property from its value or values.
• Within each ruleset, you must use a semicolon (;) to separate each declaration from the next one.

To modify multiple property values in one ruleset, write them separated by semicolons, like this:

p {
    color: red;
    width: 500px;
    border: 1px solid black;
}

You can also select multiple elements and apply a single ruleset to all of them. Separate multiple selectors by commas. For example:

p, li, h1 {
    color: red;
}

There are many different types of selectors. The examples above use element selectors, which selects all elements of a given type. But we can make more specific selections as well. Here are some of the more common types of selectors:

There are many more selectors to discover. To learn more, see the MDN Selectors guide.

Now tha we've explored some CSS fundamentals, let's improve the appearance of the example by adding more rules and information to the styles.css file.

1. First, go to Google Fonts and find one you like. Add the <link> element somewhere inside your index.html's head (anywhere between the <head> and </head> tags). It looks something like this: <link href="https://fonts.googleapis.com/css?family=Open+Sans" rel="stylesheet"> This code links your page to a style sheet that loads the Open Sans font family with your webpage.
2. Next, delete the existing rule you have in your style.css file. It was a good test, but let's not continue with lots of red text.
3. Add the following lines (shown below), replacing the font-family assignment with your font-family selection. The property font-family refers to the font(s) you want to use for text. This rule defines a global base font and font size for the whole page. SInce <HTML> is the parent element of the whole page, all elements inside it inherit the same font-size and font-family.

   html {
       font-size: 10px; /* px means "pixels": the base font size is now 10 pixels high */
       font-family: "Open Sans", sans-serif; /* this should be the rest of the output you got from Google fonts */
   }

   Note: Anything in CSS between /* and */ is a CSS comment. The browser ignores comments as it renders the code. CSS comments are a way for you to write helpful notes about your code or logic.

4. Now let's set font sizes for elements that will have text inside the HTML body (<h1>, <li>, and <p>). We'll also center the heading. Finally, let's expand the second ruleset (below) with settings for line height and letter spacing to make the body content more readable.

   h1 {
       font-size: 60px;
       text-align: center;
   }
   p, li {
       font-size: 16px;
       line-height: 2;
       letter-spacing: 1px;
   }

Adjust the px values as you like. Your work-in-progress should look similar to this:

Something you'll notice about writing CSS: a lot of it is about boxes. This inludes setting size, color and position. Most HTMl elements on your page can be thought of as boxes sitting on top of other boxes.

Photo from https://www.geograph.org.uk/photo/3418115 Copyright © Jim Barton cc-by-sal/2.0

CSS layout is mostly based on the box model. Each box taking up space on your page has properties like:

• padding, the space arround the content. In the example below, it is the space arround the paragraph text.
• border, the solid line that is just outside the padding.
• margin, the space arround the outside of the border.

In this section we also use:

• width (of an element).
• background-color, the color behind an element's content add padding.
• color, the color of an element's content (usually text).
• text-shadow sets a drop shadow on the text inside an element.
• display sets the display mode of an element. (keep reading to learn more).

To continue, let's add more CSS. Keep adding these new rules at the bottom of style.css. Experiment with changing values to see what happens.

html {
    background-color: #00539F;
}

This rule sets a background color for the entire page. Change the color code to the color you chose in Color picker tool

body {
    width: 600px;
    margin: 0 auto;
    background-color: #FF9500;
    padding: 0 20px 20px 20px;
    border: 5px solid black;
}

There are several declarations for the <body> element. Let's go through these line-by-line:

• width: 600px; This forces the body to always be 600 pixels wide.
• margin: 0 auto; When you set two values on a property like margin or padding, the first value affects the element's top and bottom side (setting it to 0 in this case); the second value affects the left and right side. (Here, auto is a special value that divides the available horizontal space evenly between left and right). You can also use one, two, three, or four values, as documented in Margin Syntax
• background-color: #FF9500; This sets the element's background color. This project uses a reddish orange for the body background color, as opposed to dark blue for the <html> element. (Feel free to experiment).
• padding: 0 20px 20px 20px; This sets four values for padding. The goal is to put some space arround the content. In this example, there is no padding on the top of the body, and 20 pixels on the right, bottom and left. The values set top, right, bottom, left, in that order. As with margin, you can use one, two, three, or four values, as documented in Padding Syntax
• border: 5px solid black; This sets values for the width, style and color of the border. In this case, it's a five-pixel-wide, solid black border, on all sides of the body.

h1 {
    margin: 0;
    padding: 20px 0;
    color: #00539F;
    text-shadow: 3px 3px 1px black;
}

You may have noticed there's a horrible gap at the top of the body. That happens because browsers apply default styling to the <h1> element (among others). That might seem like a bad idea, but the intent is to provide basic readability for unstyled pages. To eliminate the gap, we overwrite the browser's default styling with the setting margin: 0;.

Next, we set the heading's top and bottom padding to 20 pixels.

Following that, we set the heading text to be the same color as the HTML background color.

Finally, text-shadow applies a shadow to the text content of the element. Its four values are:

• The first pixel value sets the horizontal offset of the shadow from the text: how far it moves across.
• The second pixel value sets the vertical offset of the shadow from the text: how far it moves down.
• The third pixel value sets the blur radius of the shadow. A larger value produces a more fuzzy-looking shadow.
• The fourth value sets the base color of the shadow.

Try experimenting with different values to see how it changes the appearance.

img {
    display: block;
    margin: 0 auto;
}

Next, we center the image to make it look better. We could use the margin: 0 auto trick again as we did for the body. But there are differences that require an additional setting to make the CSS work.

The <body> is a block element, meaning it takes up space on the page. The margin applied to a block element will be respected by other elements on the page. In contrast, images are inline elements, for the auto margin trick to work on this image, we must give it block-level behavior using display: block;.

If you followed all the instructions in this article, you should have a page that looks similar to this one:

(You can view our version here) If you get stuck, you can always compare your work with our finished example code on GitHub.

In this exercise, we have just scratched the surface of CSS. To go further, see Learning to style HTML using CSS.

All the documentation in this page is taken from MDN