CSS hover selector for background-color not working after dynamic change of background-color with jquery

The problem you're experiencing is the importance of the location of the CSS information:

An external stylesheet is over-ruled by CSS in the head of the document; which is, in turn, over-ruled by CSS in the style attribute of the element. Basically the last style encountered by the browser overrides any previously specified, and otherwise-conflicting, rules (unless the keyword of !important is used).

As JavaScript, and therefore jQuery, places its CSS/styling information into the in-line style attribute of the element this always overrides conflicting styles specified elsewhere.

The places more importance on the color: red for the div, disregarding the div:hover styles.

To work around it, you can use:

div:hover {
    background-color: blue!important;
}

JS Fiddle demo.

A better solution, though, is to avoid assigning the background-color/other styles with jQuery, and simply use CSS:

div {
    background-color: red;
}

div:hover {
    background-color: blue!important;
}

Alternatively, you could use jQuery's hover() method:

$('div').css('background-color','red').hover(
    function(){
        $(this).css('background-color','blue');
    },
    function(){
        $(this).css('background-color','red');
    });

JS Fiddle demo.

The jquery css method adds an inline style to your elements, which means that after executing it, your div will look like

<div style="background-color: red">Hello world</div>`

Now, inline styling has always more priority than css styling, hence your problem.

So, why not adding a css class instead of using inline styling? Try with the following:

$("div").addClass("edited");

and

div:hover, div.edited:hover {
   background-color: blue;
}

div.edited {
   background-color: red;
}

and you'll see it works.

When you manipulate css with jQuery it adds it inline and overrides any css in a stylesheet try using jquery to add and remove a class that changes the color on hover. Here's the working fiddle: http://jsfiddle.net/KVmCt/6/

jQuery looks like this:

$("div").hover(
function(){
$(this).addClass("blue");
 }
,
function(){
$(this).removeClass("blue");
});