我想知道document，HTMLDocument，Document，Element和Node之间的关系，以实现IE8的getElementsByClassname方法..但每当我尝试这些方法时，它都不起作用：
document.getElementsByClassname = function(){}，HTMLDocument.getElementsByClassname=function(){}或 Element.prototype.getElementsByClassname=function(){}。

        if(!Element.getElementsByClassname) {
            Element.prototype.getElementsByClassname = function(arg){
                var cssSelector;
                var parentElement = this.document||this.documentElement.document||document;
                var queryElements = null;

                if(arg){
                    if(arg.indexOf(' ')>0){
                        cssSelector = '[class~="'+arg+'"]';
                    }else{
                        cssSelector = "."+arg;
                    }
                    queryElements = parentElement.querySelectorAll(cssSelector);
                }
                return queryElements;
            }
        }
        var elems = document.getElementsByClassname('limit').item(0);
        var triangles = elems.getElementsByClassname('triangle');
        debugger;