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In ie ,Why we can check checkbox when id="overlay" z-index : 999999?
In ie ,Why we can check checkbox when id="overlay"  z-index : 999999 ?
http://jsfiddle.net/peap/h2p9Lcg2/
<form method="post">
<input type="checkbox" name="a" value="a">A<br>
<input type="checkbox" name="b" value="b">B<br>
</form>

<div id="overlay" style="position: absolute; width: 100%; height: 100%; top: 0px; 
    left: 0px; text-align: center; padding-top: 25%; z-index: 999999999; cursor: wait; 
    /* display: none; */ background: none 0% 0% repeat scroll 
    rgba(255, 255, 255, 0.498039);">
    <img src="http://jimpunk.net/Loading/wp-content/uploads/loading83.gif">
</div>


what version of IE?? I can't check them using IE11.

i test i ie7 and 8, ie7 , 8 can check them.

Replacing the background css of the #overlay with this background will fix it for you:
background: url(data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7);
It creates a transparent gif which will prevent the click through. 
UPDATE:
You could just have easily used background: url(./Images/1x1.gif); but you would also need to create and include that .gif file and the browser would need to use an HTTP request to grab that image..., 
Using a data stream instead works just using CSS alone.

Don't replace the background with it though, place background: rgba(255, 255, 255, 0.498039); after it so that modern browsers will still have their nicer background.

wf4 -------- it's work, but i want to know what is data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7

basically, @การการการนานานายานายา it's setting the background image as a base64 data stream. If you imagine opening a .gif file to view its source in base64, this is what you would see.

and base64 is just a number series, e.g. base2 is binary (1's and 0's), base10 is 0-9.

and all browser support data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7 ?

yep, @การการการนานานายานายา :-)

In IE, the element should have background. One easy, cross browser solution is using background and opacity together:
#overlay {
    position: absolute;
    width: 100%;
    height: 100%;
    top: 0px; 
    left: 0px;
    z-index: 999999999;
    cursor: wait;

    background: white;
    opacity: 0; /* browser */
    filter: alpha(opacity=0); /* IE */
}

