 <p>I see.. Because you must change browser css defaults use in <a href="http://meyerweb.com/eric/tools/css/reset/" rel="nofollow">reset.css</a> or <a href="https://necolas.github.io/normalize.css/" rel="nofollow">normalize.css</a></p> <p>Example delete margin:</p> <p>index.html page:</p> <pre><code>&lt;html&gt; &lt;head&gt; &lt;style&gt; html, body { height:100%; margin:0px; //--&gt; added this line } &lt;/style&gt; &lt;/head&gt; &lt;body style="height:100%;"&gt; &lt;div style="width:100%; height:100%;"&gt; &lt;div id="theHead" style="height:10%; "&gt; &lt;/div&gt; &lt;div id="theMain" style="height:80%;"&gt; &lt;/div&gt; &lt;div id="theBottom" style="height:10%;"&gt; &lt;/div&gt; &lt;/div&gt; &lt;/body&gt; &lt;/html&gt; </code></pre> <p><a href="http://hg.mozilla.org/mozilla-central/file/tip/layout/style/html.css" rel="nofollow">Firefox default stylesheet file</a></p> <p><a href="http://trac.webkit.org/browser/trunk/Source/WebCore/css/html.css" rel="nofollow">Webkit default stylesheet file</a></p>