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<div class="container">
    <span class="img center absolute"></span>
</div>

<div class="container" style="margin-top: 20px;">
    <span class="img center-translate absolute"></span>
</div>

.container {
    position: relative;
    background-color: #eee;
    width: 200px;
    height: 200px;
}
.img {
    display: inline-block;
    width: 64px;
    height: 64px;
    background-color: green;
}
.absolute {
    position: absolute;
    top: 50%;
    left: 50%;
}
.center {
    margin-left: -32px;
    margin-top: -32px;
}
.center-translate {
    -webkit-transform: translate(-50%, -50%);
            transform: translate(-50%, -50%);
}