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---
language: css
contributors:
- ["Mohammad Valipour", "https://github.com/mvalipour"]
- ["Marco Scannadinari", "https://github.com/marcoms"]
- ["Geoffrey Liu", "https://github.com/g-liu"]
- ["Connor Shea", "https://github.com/connorshea"]
- ["Deepanshu Utkarsh", "https://github.com/duci9y"]
- ["Tyler Mumford", "https://tylermumford.com"]
- ["Rajat Gupta","httpd://github.com/rajat-explorer"]
filename: learncss.css
---
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](http://dabblet.com/).
The main focus of this article is on the syntax and some general tips.
## Syntax
```css
/* 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$='ue'] { font-size:smaller; }
/* or contains a value in a space-separated list */
[otherAttr~='foo'] { }
[otherAttr~='bar'] { }
/* or contains a value in a dash-separated list, ie, "-" (U+002D) */
[otherAttr|='en'] { font-size:smaller; }
/* You can combine different selectors to create a more focused selector. Don't
put spaces between them. */
div.some-class[attr$='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 */
/* 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`.
```xml
<!-- 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 occuring later in the stylesheet overwrites a previous one (which also means that if two different linked stylesheets contain rules for a block and if the rules below don't differentiate them, 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:
```css
/* A */
p.class1[attr='value']
/* B */
p.class1 { }
/* C */
p.class2 { }
/* D */
p { }
/* E */
p { property: value !important; }
```
and the following markup:
```xml
<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.
* `E` has the highest precedence because of the keyword `!important`. It is recommended that you avoid its usage.
* `F` is next, because it is an inline style.
* `A` is next, because it is more "specific" than anything else. It has 3 specifiers: The name of the element `p`, its class `class1`, an attribute `attr='value'`.
* `C` is next, even though it has the same specificity as `B`. This is because it appears after `B`.
* `B` is next.
* `D` is the last one.
## 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
* [CanIUse](http://caniuse.com) (Detailed compatibility info)
* [Dabblet](http://dabblet.com/) (CSS playground)
* [Mozilla Developer Network's CSS documentation](https://developer.mozilla.org/en-US/docs/Web/CSS) (Tutorials and reference)
* [Codrops' CSS Reference](http://tympanus.net/codrops/css_reference/) (Reference)
## Further Reading
* [Understanding Style Precedence in CSS: Specificity, Inheritance, and the Cascade](http://www.vanseodesign.com/css/css-specificity-inheritance-cascaade/)
* [Selecting elements using attributes](https://css-tricks.com/almanac/selectors/a/attribute/)
* [QuirksMode CSS](http://www.quirksmode.org/css/)
* [Z-Index - The stacking context](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context)
* [SASS](http://sass-lang.com/) and [LESS](http://lesscss.org/) for CSS pre-processing
* [CSS-Tricks](https://css-tricks.com)
|
