Reset style sheets are used to create a baseline that web designers can work from no matter which browser people are using to view the web site.

On the web, a style sheet is a file or section of code contains information about how to display elements on a web page. A style sheet tells the browser where to place certain elements, how big they should be and what colour they should be. Styles sheets are written in CSS (Cascading Style Sheets).

Whether or not the designer adds a style sheet to a web site, the web browser will still use its own default style sheet to decide how big different types of text elements should be and what sort of margins they should have etc. Each browser (Internet Explorer, Firefox, Safari, Chrome, Opera etc)  uses different default settings.

So many web designers choose to use a reset style sheet to clear out all of the default setting before applying their own styles.