body has by default 8px margins:
http://www.w3.org/TR/CSS2/sample.htmlbody {
margin: 0; /* Remove body margins */
}
Or you could use this useful Global reset
*,
*::before,
*::after {
margin: 0;
padding: 0;
box-sizing: border-box;
}
If you want something less * global:
html, body, body div, span, object, iframe, h1, h2, h3, h4, h5, h6, p, blockquote, pre, abbr, address, cite, code, del, dfn, em, img, ins, kbd, q, samp, small, strong, sub, sup, var, b, i, dl, dt, dd, ol, ul, li, fieldset, form, label, legend, table, caption, tbody, tfoot, thead, tr, th, td, article, aside, figure, footer, header, hgroup, menu, nav, section, time, mark, audio, video {
margin: 0;
padding: 0;
border: 0;
outline: 0;
font-size: 100%;
vertical-align: baseline;
background: transparent;
}