Firefox
There are two default CSS style sheets that Firefox uses to display web pages. Type these into the address bar of the browser to view them:
resource://gre/res/html.css
resource://gre/res/forms.css
Safari and Chrome
Apple’s Safari and Google’s Chrome browsers are built using an open-source layout engine known as Webkit.
Here is a link to the official WebKit source code for the default CSS style sheets of Safari and Chrome.
Internet Explorer
At this point in you will not be surprised to know that Microsoft Internet Explorer does not provide you access to it’s default CSS style sheet files.
Overriding the browsers’ default CSS style sheets
You’ll see in Firefox’s default CSS files that, by default, Firefox attaches an 8px margin on the <body> tag
body {
display: block;
margin: 8px;
}
If you don’t want an 8px margin on the <body>, then you can override that CSS setting by creating your own CSS for the <body> tag, for example:
body {
margin: 0px;
}
The same principle applies to the other major browsers.
The unofficial chart of all browser default style sheets
Here is a nice chart showing the browsers’ default CSS rules