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var css = 'some-element { some-property: value; }',
    head = document.head || document.getElementsByTagName('head')[0],
    style = document.createElement('style');

style.type = 'text/css';
if (style.styleSheet) {
  style.styleSheet.cssText = css;
} else {
  style.appendChild(document.createTextNode(css));
}

head.appendChild(style);