CSS (Cascading Style Sheets), in web development, are electronic documents with a set of rules for formatting a web page.

For example, can be used to shape:

• The colors and size of various elements of the website
• Their behavior in various actions

In the past, it was customary to configure each element of the website separately, on each separate page. THE great convenience that the use of a CSS offers, is that any changes are made only in one document and are automatically applied to all pages that use it. So a website that uses CSS, can change its appearance much more easily.

For validity check of a CSS document, based on web standards, a CSS validator can be used: https://jigsaw.w3.org/css-validator/

For creating and editing CSS documents, a simple text editor or a CSS-specific program can be used that offers several conveniences in configuring the CSS code. A program (open source, freely available to download and use) for editing CSS documents is the CSSED.