What is a CSS reset?

A CSS reset is a chunk of CSS that attempts to "reset" all styles so that a web developer can start with a plain canvas that is consistent across browsers. Having all styles uniform including heading tags makes the web site much easier and nicer to style.

Where does it go?

A CSS reset should always be the first stylesheet linked to on a web page. The reason for this is due to the order in which styles are overridden, if there are two styles for 'h1' then the second will take precedence unless the first contains the !important declaration.

Visual example

Here is some plain HTML page which has not been styled at all.

And here is the same HTML page with a CSS reset applied, as you can see all the formatting is gone.

Stylesheet

Here is an example CSS reset taken from meyerweb.com/eric/tools/css/reset/, there are many others easily accessible on the web.

/* http://meyerweb.com/eric/tools/css/reset/ 
   v2.0 | 20110126
   License: none (public domain)
*/

html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed, 
figure, figcaption, footer, header, hgroup, 
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
 margin: 0;
 padding: 0;
 border: 0;
 font-size: 100%;
 font: inherit;
 vertical-align: baseline;
}
/* HTML5 display-role reset for older browsers */
article, aside, details, figcaption, figure, 
footer, header, hgroup, menu, nav, section {
 display: block;
}
body {
 line-height: 1;
}
ol, ul {
 list-style: none;
}
blockquote, q {
 quotes: none;
}
blockquote:before, blockquote:after,
q:before, q:after {
 content: '';
 content: none;
}
table {
 border-collapse: collapse;
 border-spacing: 0;
}