CSS Rules

CSS properties consist of key-value pairs separated by a colon (:). For example:

color: red

indicates that the styled HTML elements should be given a red color.

Multiple properties are separated by semicolons (;), i.e.:

color: red;
background-color: green;

Rules are CSS properties grouped within curly braces ({}) and proceeded by a CSS selector to identify the HTML element(s) they should be applied to:

p {
  color: red;
  background-color: green;

In this example, all paragraph elements (<p>) should have red text on a green background (how festive!).

And difficult to read!

Short Forms

Some properties have multiple forms allowing for some abbreviation. For example, the CSS property:

border: 1px solid black

is a short form for three separate border-related properties:

border-width: 1px;
border-style: solid;
border-color: black;

Experimental Features and Prefixes

As new features are considered for inclusion in CSS, browsers may adopt experimental implementations. To separate these from potentially differing future interpretations, these experimental properties are typically prefixed with a browser-specific code:

  • -webkit- Webkit Browsers (Chrome, Safari, newer Opera versions, and iOS)
  • -moz- Mozilla
  • -ms- Microsoft browsers (IE, Edge)
  • -o- Older Opera versions

For example, most browsers adopted the box-shadow property before it achieved candidate status, so to use it in the Mozilla browser at that point you would use:

-moz-box-shadow: black 2px 2px 2px

To make it work for multiple browsers, and future browsers when it was officially adopted, you might use:

-webkit-box-shadow: black 2px 2px 2px;
-moz-box-shadow: black 2px 2px 2px;
-ms-box-shadow: black 2px 2px 2px;
box-shadow: black 2px 2px 2px;

The browser will ignore any properties it does not recognize, hence in Chrome 4, the -webkit-box-shadow will be used and the rest ignored, while in Chrome 10+ the box-shadow property will be used.


You should always place the not-prefixed version last, to override the prefixed version if the browser supports the official property.


The Mozilla Developer Network maintains a wiki of comprehensive descriptions of CSS properties and at the bottom of each property’s page is a table of detailed browser support. For example, the box-shadow property description can be found at: https://developer.mozilla.org/en-US/docs/Web/CSS/box-shadow. By combining the css property name and the keyword mdn in a Google search, you can quickly reach the appropriate page.