Edit in JSFiddle

$('#content').resizable();

<div id="content">
    <div id="margin-center">
        <h4>For block - <br>&emsp;margin: auto; </h4>
        <div></div>
        <div></div>
    </div>
    <div id="text-center">
        <h4>For inline block - <br>&emsp;text-align: center; </h4>
        <div></div>
        <div></div>
    </div>
    <div id="position-center">
        <h4>For absolute positioned block - <br>&emsp;margin: auto; <br>&emsp;left: 0;<br>&emsp;right: 0;</h4>
        <div></div>
    </div>
</div>

#margin-center {
    background: #CCCCFF;
}
#margin-center > div {
    background: #FFFFAA;
    border: 3px solid #FFFFFF;
    display: block;
    margin: auto;
}

#text-center {
    background: #FFCCCC;
    text-align: center;
}
#text-center > div {
    background: #AAFFFF;
    border: 3px solid #FFFFFF;
    display: inline-block;
}

#position-center {
    background: #CCFFCC;
    height: 200px;
    position: relative;
}
#position-center > div {
    background: #FFAAFF;
    border: 3px solid #FFFFFF;
    display: block;
    left: 0;
    margin: auto;
    position: absolute;
    right: 0;    
}

#content > div > div {
    height: 100px;
    width: 100px;
}
#content > div > h4 {
    display: block;
    font-size: 14px;
    margin-top: 0;
    text-align: left;
}
#content > div {
    padding: 20px 0;
}

#content {
    border: 1px dotted gray;
    display: inline-block;
    font-size: 0;
    height: auto;
    max-width: 400px;
    width: 100%;
}