<style>
#a:
{
color: blue;
}
</style>
<style>
#a:
{
background: gray;
}
</style>
<script>
function addCSSRule(key,value){
var css = document.styleSheets[document.styleSheets.length-1];
css.cssRules ?
(css.insertRule(key+"{"+value+"}", css.cssRules.length)) :
(css.addRule(key,value)) ;
}
function removeCSSRule(key){
for(var i = 0; i < document.styleSheets.length; i++){
var css = document.styleSheets[i];
css.cssRules ?
(function(){
for(var j = 0; j < css.cssRules.length; j++){
if(css.cssRules[j].selectorText==key){
css.deleteRule(j);
}
}
})() :
(css.removeRule(key)) ;
}
}
</script>
<div id="a">
abc
</div>
<button onclick='addCSSRule("#a","color:red;background:yellow;")'>
add</button>
<button onclick='removeCSSRule("#a")'>
remove</button>