How to detect a browser that dosen't render .png transparency

Question

I have this code that render's a image acording to the day of the week. But in IE6 and lower and probably some other browsers it won't render png opacity. So I want to change it a litle so that it will detect the browser's that don't render alpha transparency and tell them to load this image instead: "img/horarios2.png". Ive tried making it so that it would rule out IE6 and lower that are known for not rendering, but then I thinking about all the other browsers that I probably don't know about that don't render either and needed something that would rule them out also. I don't know the best way to acomplish this so im open to all sugestions.

$(document).ready (function horario () {
var date = new Date();
var weekday = (date.getDay());

if (weekday==0)
    document.getElementById('horarios').src = "img/domingo.png";
else if (weekday==4)
    document.getElementById('horarios').src = "img/quinta.png";
else if (weekday==5)
    document.getElementById('horarios').src = "img/sexta.png";
else if (weekday==6)
    document.getElementById('horarios').src = "img/sabado.png";
else 
    document.getElementById('horarios').src = "img/quarta.png";
});

Answer 1

I've done a reasonable amount of research into this this issue and came to the conclusion that it was not feasible to create a feature test in JavaScript, and that IE 6 is the only browser in serious use that doesn't support PNG transparency. That being the case, your best bet is conditional comments, as recommended by Gabriel Ross. It is possible to use conditional comments in pure JavaScript, which I think is what you want:

var div = document.createElement("div");
div.innerHTML = "<!--[if lte IE 6]><i></i><![endif]-->";
var isIe6orLower = (div.getElementsByTagName("i").length == 1);

alert("Is IE 6 or lower: " + isIe6orLower);

UPDATE 17 June 2012

Note that IE 10 will not support conditional comments, so this approach will not work in future versions of IE.