Copiar código del código de la siguiente manera:
Window.alert = function (STR)
{{
var shield = document.createElement ("div");
Shield.id = "Shield";
Shield.style.Position = "Absolute";
shield.style.left = "0px";
shield.style.top = "0px";
Shield.style.width = "100%";
shield.style.height = document.body.scrollheight+"px";
// El color de fondo cuando aparece el cuadro de diálogo
shield.style.background = "#fff";
shield.style.textalign = "centro";
shield.style.zindex = "25";
// Antecedentes transparentes, IE, es válido
//shield.style.filter = "alfa (opacidad = 0)";
var alertfram = document.createElement ("div");
alertfram.id = "alertfram";
alertfram.style.position = "Absolute";
alertfram.style.left = "50%";
alertfram.style.top = "50%";
alertfram.style.marginleft = "-225px";
alertfram.style.margIntop = "-75px";
alertfram.style.width = "450px";
alertfram.style.height = "150px";
alertfram.style.background = "#FF0000";
alertfram.style.textalign = "Center";
alertfram.style.lineheight = "150px";
alertfram.style.zindex = "300";
strhtml = "<ul style =/" list-style: Ninguno;
STRHTML += "Li Style =" 1px sólido #f9cade;/"> [indicador personalizado] </li>/n";
STRHTML += "Li Style =" /">"+str+"</li>/n";
STRHTML += "<li style =" /"Botón/" valor =/"ok/" onClick =/"dook ()/"/> </li>/n ";
strhtml += "</ul>/n";
alertfram.innerhtml = strhtml;
document.body.appendChild (alertfram);
document.body.appendChild (escudo);
var adj = setInterval ("doalpha ()", 5);
this.dook = function () {
alertfram.style.display = "Ninguno";
escudo.
}
alertfram.focus ();
document.body.onselectstart = function () {return false;};
}
El efecto es como se muestra en la figura