How to remove margin space around body or clear default css styles

body has by default 8px margins: http://www.w3.org/TR/CSS2/sample.html

body { 
    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;
}

some other CSS Reset:

http://meyerweb.com/eric/tools/css/reset/
https://github.com/necolas/normalize.css/
http://html5doctor.com/html-5-reset-stylesheet/
…