function addCSSRule(selector, css) { var sheets = document.styleSheets, sheet = sheets[sheets.length - 1]; if(sheet.insertRule) { sheet.insertRule(selector + '{' + css + '}', sheet.cssRules.length); }else if(sheet.addRule) { sheet.addRule(selector, css, -1); } } addCSSRule('hoge:after', 'background: red');