/** 
* Add a stylesheet rule to the document (may be better practice, however, 
* to dynamically change classes, so style information can be kept in 
* genuine styesheets (and avoid adding extra elements to the DOM)) 
* Note that an array is needed for declarations and rules since ECMAScript does 
* not afford a predictable object iteration order and since CSS is 
* order-dependent (i.e., it is cascading); those without need of 
* cascading rules could build a more accessor-friendly object-based API. 
* @param {Array} decls Accepts an array of JSON-encoded declarations 
* @example 
addStylesheetRules([ 
['h2', // Also accepts a second argument as an array of arrays instead 
['color', 'red'], 
['background-color', 'green', true] // 'true' for !important rules 
], 
['.myClass', 
['background-color', 'yellow'] 
] 
]); 
*/ 
function addStylesheetRules (decls) { 
var style = document.createElement('style'); 
document.getElementsByTagName('head')[0].appendChild(style); 
if (!window.createPopup) { /* For Safari */ 
style.appendChild(document.createTextNode('')); 
} 
var s = document.styleSheets[document.styleSheets.length - 1]; 
for (var i=0, dl = decls.length; i < dl; i++) { 
var j = 1, decl = decls[i], selector = decl[0], rulesStr = ''; 
if (Object.prototype.toString.call(decl[1][0]) === '[object Array]') { 
decl = decl[1]; 
j = 0; 
} 
for (var rl=decl.length; j < rl; j++) { 
var rule = decl[j]; 
rulesStr += rule[0] + ':' + rule[1] + (rule[2] ? ' !important' : '') + ';\n'; 
} 
if (s.insertRule) { 
s.insertRule(selector + '{' + rulesStr + '}', s.cssRules.length); 
} 
else { /* IE */ 
s.addRule(selector, rulesStr, -1); 
} 
} 
} 

addStylesheetRules(["div.content", ["color": "#000"], ["border-width","1px"], ["border-style", "solid"]]) 

<style> 
div.content{color:#000;border:1px solid} 
</style 

addStylesheetRules( 
[selector, [attr, value], …], 
[selector, [attr, value], …] 
);