If you're injecting CSS, here's a cross-browser (not thoroughly tested, though) way to do it:
Code:
if (document.createStyleSheet) {
    var style = document.createStyleSheet();
    style.addRule("div.content", "display: none;");
} else {
    var head = document.getElementsByTagName("head")[0];
    if (head) {
        var style = document.createElement("style");
        style.setAttribute("type", "text/css");
        style.appendChild(document.createTextNode("div.content { display: none; }"));
        head.appendChild(style);
    }
}
I used the above code to hide elements in a script that was located in the <head> tag (at which point no elements are programmatically accessible.)