javascript - Body of page not shrinking at all, CSS, HTML -

right have body width of 90%, , left , right margin of 5%, , border of 1px. here's i'm trying do. i'm trying make when zoom out way using ctr-, it'll shrink until it's pin-prick in middle of page, right body's width isn't shrinking @ when zoom out way. it's staying same size. not sure did wrong. please take @ code.

body { width: 90%; margin-left: 5%; margin-right: 5%; border: 1px solid black; } 

p.s: if you're not understanding i'm talking about, please either hit ctr-, or ctr , scroll thing on mouse until page's size around 25%. see how stackoverflow pin-prick smack-dab in middle? see width of entire site being shrunk down size? well, when that, body of site doesn't shrink @ all. stays same size.

change % px

body { width: 1000px; margin-left: 5%; margin-right: 5%; border: 1px solid black;  margin:auto; }