Cascading style sheets (.css) are used in website development to separate page presentation from a document's text content.

Despite the .css extension, these files are plain text and can be edited in any plain text editor.

Cascading style sheets are written in mark-up language where each CSS parameter has a corresponding HTML element. These parameters can then over-ride the HTML elements default display values, allowing a developer to change values such as font style, size and color to borders and line-spacing.

For example, a standard HTML paragraph element such as:

<p>This text is my paragraph</p>

Would be styled using the following cascading style sheet instruction:

p { color: blue; font-size: 20px; }

One of the main advantages of using cascading style sheets is that design instructions for an entire website can be consolidated in to a single file, reducing the amount of work required to make site-wide changes to a website.