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Web pages are built with HTML, which specifies the content of a page. CSS (Cascading Style Sheets) is a separate language which specifies a page’s appearance.

CSS code is made of static rules. Each rule takes one or more selectors and gives specific values to a number of visual properties. Those properties are then applied to the page elements indicated by the selectors.

This guide has been written with CSS 2 in mind, which is extended by the new features of CSS 3.

NOTE: Because CSS produces visual results, in order to learn it, you need to try everything in a CSS playground like dabblet. The main focus of this article is on the syntax and some general tips.

Syntax

/* comments appear inside slash-asterisk, just like this line!
   there are no "one-line comments"; this is the only comment style */

/* ####################
   ## SELECTORS
   #################### */

/* the selector is used to target an element on a page. */
selector { property: value; /* more properties...*/ }

/*
Here is an example element:

<div class='class1 class2' id='anID' attr='value' otherAttr='en-us foo bar' />
*/

/* You can target it using one of its CSS classes */
.class1 { }

/* or both classes! */
.class1.class2 { }

/* or its name */
div { }

/* or its id */
#anID { }

/* or using the fact that it has an attribute! */
[attr] { font-size:smaller; }

/* or that the attribute has a specific value */
[attr='value'] { font-size:smaller; }

/* starts with a value (CSS 3) */
[attr^='val'] { font-size:smaller; }

/* or ends with a value (CSS 3) */
[attr$='ue'] { font-size:smaller; }

/* or contains a value in a space-separated list */
[otherAttr~='foo'] { }
[otherAttr~='bar'] { }

/* or contains a value in a dash-separated list, e.g., "-" (U+002D) */
[otherAttr|='en'] { font-size:smaller; }


/* You can combine different selectors to create a more focused selector. Don't
   put spaces between them. */
div.some-class[attr$='ue'] { }

/* You can select an element which is a child of another element */
div.some-parent > .class-name { }

/* or a descendant of another element. Children are the direct descendants of
   their parent element, only one level down the tree. Descendants can be any
   level down the tree. */
div.some-parent .class-name { }

/* Warning: the same selector without a space has another meaning.
   Can you guess what? */
div.some-parent.class-name { }

/* You may also select an element based on its adjacent sibling */
.i-am-just-before + .this-element { }

/* or any sibling preceding it */
.i-am-any-element-before ~ .this-element { }

/* There are some selectors called pseudo classes that can be used to select an
   element only when it is in a particular state */

/* for example, when the cursor hovers over an element */
selector:hover { }

/* or a link has been visited */
selector:visited { }

/* or hasn't been visited */
selected:link { }

/* or an element is in focus */
selected:focus { }

/* any element that is the first child of its parent */
selector:first-child {}

/* any element that is the last child of its parent */
selector:last-child {}

/* Just like pseudo classes, pseudo elements allow you to style certain parts of
    a document  */

/* matches a virtual first child of the selected element */
selector::before {}

/* matches a virtual last child of the selected element */
selector::after {}

/* At appropriate places, an asterisk may be used as a wildcard to select every
   element */
* { } /* all elements */
.parent * { } /* all descendants */
.parent > * { } /* all children */

/* ####################
   ## PROPERTIES
   #################### */

selector {

    /* Units of length can be absolute or relative. */

    /* Relative units */
    width: 50%;       /* percentage of parent element width */
    font-size: 2em;   /* multiples of element's original font-size */
    font-size: 2rem;  /* or the root element's font-size */
    font-size: 2vw;   /* multiples of 1% of the viewport's width (CSS 3) */
    font-size: 2vh;   /* or its height */
    font-size: 2vmin; /* whichever of a vh or a vw is smaller */
    font-size: 2vmax; /* or greater */

    /* Absolute units */
    width: 200px;     /* pixels */
    font-size: 20pt;  /* points */
    width: 5cm;       /* centimeters */
    min-width: 50mm;  /* millimeters */
    max-width: 5in;   /* inches */

    /* Colors */
    color: #F6E;                 /* short hex format */
    color: #FF66EE;              /* long hex format */
    color: tomato;               /* a named color */
    color: rgb(255, 255, 255);   /* as rgb values */
    color: rgb(10%, 20%, 50%);   /* as rgb percentages */
    color: rgba(255, 0, 0, 0.3); /* as rgba values (CSS 3) Note: 0 <= a <= 1 */
    color: transparent;          /* equivalent to setting the alpha to 0 */
    color: hsl(0, 100%, 50%);    /* as hsl percentages (CSS 3) */
    color: hsla(0, 100%, 50%, 0.3); /* as hsl percentages with alpha */

    /* Borders */
    border-width:5px;
    border-style:solid;
    border-color:red;      /* similar to how background-color is set */
    border: 5px solid red; /* this is a short hand approach for the same */
    border-radius:20px;    /* this is a CSS3 property */    

    /* Images as backgrounds of elements */
    background-image: url(/img-path/img.jpg); /* quotes inside url() optional */

    /* Fonts */
    font-family: Arial;
    /* if the font family name has a space, it must be quoted */
    font-family: "Courier New";
    /* if the first one is not found, the browser uses the next, and so on */
    font-family: "Courier New", Trebuchet, Arial, sans-serif;
}

Usage

Save a CSS stylesheet with the extension .css.

<!-- You need to include the css file in your page's <head>. This is the
     recommended method. Refer to http://stackoverflow.com/questions/8284365 -->
<link rel='stylesheet' type='text/css' href='path/to/style.css'>

<!-- You can also include some CSS inline in your markup. -->
<style>
   a { color: purple; }
</style>

<!-- Or directly set CSS properties on the element. -->
<div style="border: 1px solid red;">
</div>

Precedence or Cascade

An element may be targeted by multiple selectors and may have a property set on it in more than once. In these cases, one of the rules takes precedence over others. Rules with a more specific selector take precedence over a less specific one, and a rule occurring later in the stylesheet overwrites a previous one (which also means that if two different linked stylesheets contain rules for an element and if the rules are of the same specificity, then order of linking would take precedence and the sheet linked latest would govern styling) .

This process is called cascading, hence the name Cascading Style Sheets.

Given the following CSS:

/* A */
p.class1[attr='value']

/* B */
p.class1 { }

/* C */
p.class2 { }

/* D */
p { }

/* E */
p { property: value !important; }

and the following markup:

<p style='/*F*/ property:value;' class='class1 class2' attr='value'>

The precedence of style is as follows. Remember, the precedence is for each property, not for the entire block.

Media Queries

CSS Media Queries are a feature in CSS 3 which allows you to specify when certain CSS rules should be applied, such as when printed, or when on a screen with certain dimensions or pixel density. They do not add to the selector’s specificity.

/* A rule that will be used on all devices */
h1 {
  font-size: 2em;
  color: white;
  background-color: black;
}

/* change the h1 to use less ink on a printer */
@media print {
  h1 {
    color: black;
    background-color: white;
  }
}

/* make the font bigger when shown on a screen at least 480px wide */
@media screen and (min-width: 480px) {
  h1 {
    font-size: 3em;
    font-weight: normal;
  }
}

Media queries can include these features: width, height, device-width, device-height, orientation, aspect-ratio, device-aspect-ratio, color, color-index, monochrome, resolution, scan, grid. Most of these features can be prefixed with min- or max-.

The resolution feature is not supported by older devices, instead use device-pixel-ratio.

Many smartphones and tablets will attempt to render the page as if it were on a desktop unless you provide a viewport meta-tag.

<head>
  <meta name="viewport" content="width=device-width; initial-scale=1.0">
</head>

Compatibility

Most of the features in CSS 2 (and many in CSS 3) are available across all browsers and devices. But it’s always good practice to check before using a new feature.

Resources

Further Reading


Got a suggestion? A correction, perhaps? Open an Issue on the Github Repo, or make a pull request yourself!

Originally contributed by Mohammad Valipour, and updated by 21 contributor(s).