Slight rule optimisation: background-size could be put into background rule

[siers]

from https://developer.mozilla.org/en-US/docs/Web/CSS/background

<bg-layer> = <bg-image> || <bg-position> [ / <bg-size> ]? || <repeat-style> || <attachment> || <box> || <box>

So instead of

    background: url('icons-retina.png') no-repeat top left;
    background-size: 209px 2254px;

you could technically probably do background: url('icons-retina.png') no-repeat top left / 209px 2254px;, I think.