The .js seems to allow for either. I would speculate (after no more than a cursory glance) that using the xmp tag is the Google-indexable method of writing you

The .js seems to allow for either. I would speculate (after no more than a cursory glance) that using the xmp tag is the Google-indexable method of writing your page? Google should see it as plain text on a web page. Textarea support could be to provide interactive/editable pages?