Trivial Tips

Avoid double loading

#header {
    background: no-repeat center #fff;
    background-size: cover;
}
@media only screen and (-webkit-max-device-pixel-ratio: 1),
       only screen and (-o-max-device-pixel-ratio: 1/1),
       only screen and (max-resolution: 1dppx),
       only screen and (max-resolution: 96dpi) {
    #header {
        background-image: url(small.jpg);
    }
}
@media only screen and (-webkit-min-device-pixel-ratio: 2),
       only screen and (-o-min-device-pixel-ratio: 2/1),
       only screen and (min-resolution: 2dppx),
       only screen and (min-resolution: 192dpi) {
    #header {
        background-image: url(small-2x.jpg);
    }
}

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