I am looking for a method like document.getElementsByClassName(), but just for attributes, that would also mean that document.getElementsByAttributeValue("class", "testclass") would have the same output as document.getElementsByClassName("testclass").
It is not that easy since you cannot just add a function like this
document.getElementsByAttributeValue = function(){....};
because it has to work for document.getElementById("foo").getElementsByAttributeValue(...) as well.
Needless to say, that I don't want a JQuery solution because there is no need for a giant library for just one function.