document.load() is a part of an old version of the W3C DOM Level 3 Load & Save module. Can be used with document.async to indicate whether the request is synchronous or asynchronous (the default). As of at least Gecko 1.9, this no longer supports cross-site loading of documents (Use XMLHttpRequest instead).
See also the load sample in the XML tests directory. (To test this functionality, create the files on your local disk or on a webserver rather than loading the load.html file from the LXR-generated page, which will serve the text.xml file as HTML.)