toHtml() ----- 将节点树输出为html文件

        function toHTML(node){
            var result = '';
            if(node.nodeType == 1){
                var tagName = node.tagName.toLowerCase(),
                    attrs = node.attributes,
                    attrStr = ' ';
                    if(attrs.length){
                        for(var i = 0; i < attrs.length; i++){
                            attrStr += attrs[i].nodeName + '="' + attrs[i].nodeValue + '"  ';
                        }
                    }
                var open = '<' + tagName + attrStr + '>';
                var close = '</' + tagName + '>';
                var children = node.childNodes;

                if(children.length == 0){
                    result = open + close;
                }else{
                    for(var i = 0; i < children.length; i++){
                        result += toHTML(children[i]);
                    }
                    result = open + result + close;
                }
            }else if(node.nodeType == 8){
                result += '<!--' + node.nodeValue + '-->';
            }else if(node.nodeType == 3){
                result += node.nodeValue;
            }
            return result;
        }