ModalPopupExtender AJAX flashes when page loads

ModalPopupExtender AJAX, I don't quite understand it and it likes to do stuff I don't know where it comes from.

For instance, in a project having a pop up on the master page and switching between pages the darn pop up flashes when page loads.  

solution:  add the style="display: none"

 

more at this link another-modal-popup-extender-question

css attribute coding convention

define outside attributes then inner ones last.

margin / border / background / padding / height × width

{
    display: ;    /* Where and how the box is placed */
    position: ;
    float: ;
    clear: ;

    visibility: ; /* Can the box be seen? */
    opacity: ;
    z-index: ;

    margin: ;     /* Layers of the box model */
    outline: ;
    border: ;
    background: ;
    padding: ;

    width: ;      /* Content dimensions and scrollbars */
    height: ;
    overflow: ;

    color: ;      /* Text */
    text: ;
    font: ;
}

http://rhodesmill.org/brandon/2011/concentric-css/