Pourquoi les styles CSS doivent-ils être initialisés ?

body, p, dl, dt, dd, ul, ol, li, h1, h2, h3, h4, h5, h6, pre, form, fieldset, input, p, blockquote, th, td {      
    padding:0;
    margin:0; 
}

ol, ul { 
    list-style:none; 
}
a { 
    text-decoration:none; 
}
a:hover { 
    text-decoration:underline;
 }
fieldset, img { 
    border:0; 
}
h1, h2, h3, h4, h5, h6{ 
    font-size:100%; 
}
button, input, select, textarea { 
    font-size:100%;
 }

body, h1, h2, h3, h4, h5, h6, hr, p, blockquote, dl, dt, dd, ul, ol, li, pre, form, fieldset, legend, button, input, textarea, th, td { margin:0; padding:0; }
body, button, input, select, textarea { font:12px/1.5tahoma, arial, \5b8b\4f53; }
h1, h2, h3, h4, h5, h6{ font-size:100%; }
address, cite, dfn, em, var { font-style:normal; }
code, kbd, pre, samp { font-family:couriernew, courier, monospace; }
small{ font-size:12px; }
ul, ol { list-style:none; }
a { text-decoration:none; }
a:hover { text-decoration:underline; }
sup { vertical-align:text-top; }
sub{ vertical-align:text-bottom; }
legend { color:#000; }
fieldset, img { border:0; }
button, input, select, textarea { font-size:100%; }
table { border-collapse:collapse; border-spacing:0; }