You can see what that looks like right now on http://tmbg.duckduckgo.com -- seems a bit too distracting to me, and document may imply more of a document than a Web page. Something more subtle/less specific/more generic?

It's easy to use an icon with less contrast:
Both Firefox and Chrome use this kind of icon for sites without a favicon.
A sheet of paper is also commonly used as an icon for unidentified file types.

A globe is often used to symbolise the Internet and could be used by itself or with the document base:
   

One could use a chains or something which looks a bit like a webpage , but that would feel... weird?

A simple square would be fine, too.

Of course there could be better ideas, these are just some quick thoughts.

Hahaha, oh wow. It logged me out while I was typing.

The light contrast ones seem to work the best. I have it up on tmbg right now. Let me know if you want me to try anything else.

It's better than the icon with more contrast or no icon at all.
It doesn't draw too much attention and says nothing more than 'this is a link without a real favicon'.

And uh, the icon is stolen from the Fugue set (I just picked it because I have a copy of the whole set uploaded) which is licensed under Creative Commons Attribution 3.0.

I could just draw a custom one to avoid having to put a link back (after all, it's just a sheet of paper).

Cool--I think it is worth experimenting with on on the live site. 

As subtle as it can get: or

That actually looks pretty good--up on tmbg now. Except for that it clashes with the Chrome default when the image breaks. There is currently a bug in the provider that cause some to break, so for now I wonder if we should just take that chrome default and run with it.

Chrom.

That's on tmbg now.