Edit in JSFiddle

var renderTemplate = function (tpl, dict) {
    if (!dict) return tpl;
    for (var key in dict) {
        tpl = tpl.replace(
            new RegExp("{"+key+"}", "g"), 
            dict[key]
        );
    }
    return tpl;
};

// usage example:
document.getElementsByTagName('body')[0].innerHTML = 
    renderTemplate("<p class={class}>{content}</p>", {
        class: "foo",
        content: "lorem ipsum dolor"
    });