myLabel.getAttribute("for");

(where “myLabel” is a reference to a <label> node)
The function always returns NULL, even if the “for” attribute is manually specified into HTML code, ie:

<label for="my_field_id">my label</label>

Fortunately, after a brief search on Google, I’ve found a post on quirksmode.org, which contains a comment by Tino Zijdel, that show an easy and crossbrowser solution to get the “for” attribute (that can be used both to set a value or to get it):

myLabel.htmlFor

Thank you Tino!

In my search I’d even discovered that on Internet Explorer getAttribute() is different from other browsers, because it offers an extra argument called “iFlags” that is a number that can be 0, 1 or 2 (genial!) and means (I’m reporting the official microsoft documentation):

0 -> Default. Performs a property search that is not case-sensitive, and returns an interpolated value if the property is found
1 -> Performs a case-sensitive property search. To find a match, the uppercase and lowercase letters in AttributeName must exactly match those in the attribute name. If the iFlags parameter for getAttribute is set to 1 and this option is set to 0 (default), the specified property name might not be found.
2 -> Returns the value exactly as it was set in script or in the source document

It would be a better world without Internet Explorer!