于是就测试了下：
代码如下:
var stringToDom=function(text) {
var doc;
if(window.ActiveXObject) {
doc = new ActiveXObject("MSXML2.DOMDocument");
doc.loadXML(text).documentElement;
} else {
doc = (new DOMParser()).parseFromString(text,"text/xml");
}
return doc;
}
var xmlDoc=stringToDom("ab"),
c,
d1=new Date();
for(var i=0;i<100000;i++){
c=xmlDoc.getElementsByTagName("a");
}
document.write("getElementsByTagName: ",new Date()-d1);
d1=new Date();
try{
for(var i=0;i<100000;i++){
c=xmlDoc.selectNodes("a");
}
document.write("
selectNodes: ",new Date()-d1);
}catch(ex){document.write("
error:"+ex)}
在IE下selectNodes还是快多了，
可以FF下却没有这个方法，google了下，找了方法，使用XPathEvaluator来实现，下面是具体实现，不过效率就不太理想了：
代码如下:
if (!window.ActiveXObject) {
(function(){
var oEvaluator=new XPathEvaluator(),oResult;
XMLDocument.prototype.selectNodes = function(sXPath) {
oResult = oEvaluator.evaluate(sXPath, this, null, XPathResult.ORDERED_NODE_ITERATOR_TYPE, null);
var aNodes = [];
if (oResult != null) {
var oElement = oResult.iterateNext();
while (oElement) {
aNodes[aNodes.length]=oElement;
oElement = oResult.iterateNext();
}
}
return aNodes;
}
})()
}
evaluate(xpathExpression, contextNode, namespaceResolver, resultType, result);
Returns an XPathResult based on an XPath expression and other given parameters.
xpathExpression is a string representing the XPath to be evaluated.
contextNode specifies the context node for the query (see the [http://www.w3.org/TR/xpath XPath specification). It's common to pass document as the context node.
namespaceResolver is a function that will be passed any namespace prefixes and should return a string representing the namespace URI associated with that prefix. It will be used to resolve prefixes within the XPath itself, so that they can be matched with the document. null is common for HTML documents or when no namespace prefixes are used.
resultType is an integer that corresponds to the type of result XPathResult to return. Use named constant properties, such as XPathResult.ANY_TYPE, of the XPathResult constructor, which correspond to integers from 0 to 9.
result is an existing XPathResult to use for the results. null is the most common and will create a new XPathResult
完整的测试页面：
代码如下:
selectNodes&getElementsByTagName 
