Which means what? Well, it ascertains that the page loaded is HTML, which people tend to take for granted, but which is not the case for all pages on the web. Especially, it is not the case for text/plain pages, by the broad masses more commonly known as *.txt. In Firefox, text documents get rendered much like HTML pages, but in a <pre> encasing.if( !(document.contentType||'').match( /html/i ) ) return;
Saving the document will remove this "HTML enclosure", but if your script injected some other junk, like an interface of some sort with some text, for instance, the saved page also will. This is probably not what you wanted. It is at the very least certainly not what unsuspecting users of the script wanted.
Edit: a modern GM+Firefox combination gets to run at XHTML pages. (This has not always been the case. Thanks for the correction, Rod McGuire!)
But please do decorate your scripts with the above line. It's royalty and patent free software with an irrevocable, DRM free license for all time. Public domain, at its best, working for you. Cheers!