It appears that something with my caching configuration choked earlier today and served a compressed file for download to visitors in lieu of the website. This happened back in January, albeit in that case I diagnosed the issue as having compress page settings in my .htaccess file as well as WP Super Cache. In this case, I only had it set up in WP Super Cache, so I am unsure why the site behaved in the way it did. In any event, I have turned off all page compression for the time being while I try to figure out what went wrong yesterday. The site should now work as expected again.