When you are debugging the CSS, there will be times when you will see "user agent stylesheet".

So what is this "user agent stylesheet"? 

These are the CSS values generated by your Web Browser. Whenever the values of properties are not defined, Web browser tries to define them for you. 

If you don't want the defaults then there are two ways to get rid of them, which are as follows:-

1. Explictly define the values of the propertyies in your CSS StyleSheets. 

2. Try including normalize.css ( Click here to download it ) or reset.css ( Click here to see it ). Reset.css resets everything and you will have to define values of each property. Normalize.css preserve the important ones and is very nicely documented. To understand the difference between these two, Click here.