for(i=0;i<document.forms.length ;i++)
{
    for(j=0;j<document.forms[i].elements.length;j++)
    {
        if(document.forms[i].elements[j].type=="button" || document.forms[i].elements[j].type=="submit" || document.forms[i].elements[j].type=="reset")
            document.forms[i].elements[j].className="input_button";
    }
}