I've already did margin and padding to 0 but can't get it.
I'm a beginner in css and html and I know this question has been asked before but I've already seen the questions and none of them helped. This is the JSfiddle. JsFiddle
   ------< HTML Code---->
 <div id = "header">
        <h1>{Title}</h1>
        {block:Description}
            <p id="description">{Description}</p>
        {/block:Description}
</div>
        <ol id="posts">
            {block:Posts}
                {block:Text}
                    <li class="post text">
                        {block:Title}
                            <h3><a href="{Permalink}">{Title}</a></h3>
                        {/block:Title}
                        {Body}
                    </li>
                {/block:Text}
                {block:Photo}
                    <li class="post photo">
                        <img src="{PhotoURL-500}" alt="{PhotoAlt}"/>
                        {block:Caption}
                            <div class="caption">{Caption}</div>
                        {/block:Caption}
                    </li>
                {/block:Photo}
                {block:Panorama}
                    <li class="post panorama">
                        {LinkOpenTag}
                            <img src="{PhotoURL-Panorama}" alt="{PhotoAlt}"/>
                        {LinkCloseTag}
                        {block:Caption}
                            <div class="caption">{Caption}</div>
                        {/block:Caption}
                    </li>
                {/block:Panorama}
                {block:Photoset}
                    <li class="post photoset">
                        {Photoset-500}
                        {block:Caption}
                            <div class="caption">{Caption}</div>
                        {/block:Caption}
                    </li>
                {/block:Photoset}
                {block:Quote}
                    <li class="post quote">
                        "{Quote}"
                        {block:Source}
                            <div class="source">{Source}</div>
                        {/block:Source}
                    </li>
                {/block:Quote}
                {block:Link}
                    <li class="post link">
                        <a href="{URL}" class="link" {Target}>{Name}</a>
                        {block:Description}
                            <div class="description">{Description}</div>
                        {/block:Description}
                    </li>
                {/block:Link}
                {block:Chat}
                    <li class="post chat">
                        {block:Title}
                            <h3><a href="{Permalink}">{Title}</a></h3>
                        {/block:Title}
                        <ul class="chat">
                            {block:Lines}
                                <li class="{Alt} user_{UserNumber}">
                                    {block:Label}
                                        <span class="label">{Label}</span>
                                    {/block:Label}
                                    {Line}
                                </li>
                            {/block:Lines}
                        </ul>
                    </li>
                {/block:Chat}
                {block:Video}
                    <li class="post video">
                        {Video-500}
                        {block:Caption}
                            <div class="caption">{Caption}</div>
                        {/block:Caption}
                    </li>
                {/block:Video}
                {block:Audio}
                    <li class="post audio">
                        {AudioPlayerBlack}
                        {block:Caption}
                            <div class="caption">{Caption}</div>
                        {/block:Caption}
                    </li>
                {/block:Audio}
            {/block:Posts}
        </ol>
    <------CSS----------->
body { margin: 0px; padding: 0px;}
#header {background-color:red;}