The source code

    /**
     * @ignore
     *  mask extension for kissy
     * @author yiminghe@gmail.com
     */
    KISSY.add("overlay/extension/mask-render", function (S, Node) {

        var UA = S.UA,
            ie6 = (UA['ie'] === 6),
            $ = Node.all;

        function docWidth() {
            return  ie6 ? ("expression(KISSY.DOM.docWidth())") : "100%";
        }

        function docHeight() {
            return ie6 ? ("expression(KISSY.DOM.docHeight())") : "100%";
        }

        function initMask(self) {
            var maskCls = self.get("prefixCls") + "ext-mask " + self.getCssClassWithState('-mask'),
                mask = $("<div " +
                    " style='width:" + docWidth() + ";" +
                    "left:0;" +
                    "top:0;" +
                    "height:" + docHeight() + ";" +
                    "position:" + (ie6 ? "absolute" : "fixed") + ";'" +
                    " class='" +
                    maskCls +
                    "'>" +
                    (ie6 ? "<" + "iframe " +
                        "style='position:absolute;" +
                        "left:" + "0" + ";" +
                        "top:" + "0" + ";" +
                        "background:red;" +
                        "width: expression(this.parentNode.offsetWidth);" +
                        "height: expression(this.parentNode.offsetHeight);" +
                        "filter:alpha(opacity=0);" +
                        "z-index:-1;'></iframe>" : "") +
                    "</div>")
                    .prependTo("body");
            /*
             点 mask 焦点不转移
             */
            mask['unselectable']();
            mask.on("mousedown", function (e) {
                e.preventDefault();
            });
            return mask;
        }

        function Mask() {
        }

        Mask.ATTRS = {

            mask: {
                value: false
            },
            maskNode: {

            }

        };

        Mask.prototype = {

            __renderUI: function () {
                var self = this;
                if (self.get('mask')) {
                    self.set('maskNode', initMask(self));
                }
            },

            __syncUI: function () {
                var self = this;
                if (self.get('mask')) {
                    self.ksSetMaskVisible(self.get('visible'), 1);
                }
            },

            ksSetMaskVisible: function (shown, hideInline) {
                var self = this,
                    shownCls = self.getCssClassWithState('-mask-shown'),
                    maskNode = self.get('maskNode'),
                    hiddenCls = self.getCssClassWithState('-mask-hidden');
                if (shown) {
                    maskNode.removeClass(hiddenCls).addClass(shownCls);
                } else {
                    maskNode.removeClass(shownCls).addClass(hiddenCls);

                }
                if (!hideInline) {
                    maskNode.css('visibility', shown ? 'visible' : 'hidden');
                }
            },

            __destructor: function () {
                var self = this, mask;
                if (mask = self.get("maskNode")) {
                    mask.remove();
                }
            }

        };

        return Mask;
    }, {
        requires: ["node"]
    });