function change_font(what){ 

    var start_of_id = what.substr(0, 2);    /*find out which group of ids it belongs to, for example, "mk" */

    var allSpans = document.getElementsByTagName("span");   // get all span elements
    for(i=0; i < allSpans.length; i++)    // loop through all the span elements
    {
        // identify the ones with starting with an id the same as the one that triggered the event
        if (allSpans[i].id.substr(0, 2) == start_of_id)
        {
            // reset any span elements starting with id="mk" to normal text
            allSpans[i].style.fontWeight = "normal";
            allSpans[i].style.color= "#000";
            allSpans[i].style.fontSize = "12";
        }
    }
    /*  Now all have normal text 
        Next customise the span element with id that was passed to function */
    document.getElementById(what).style.fontSize = "12"; 
    document.getElementById(what).style.fontWeight = "bold";
    document.getElementById(what).style.color = "#990000";
}