[Rivet] rivet-mkhtml output requires network access

[David Grellscheid]

> Is MathJax intelligent enough to time out immediately if you set the
> browser in offline mode? (or, even better and I think the case in
> sufficiently friendly OSes, for that to *automatically* happen when
> there's no network connection.) It seems to me that whether or not
> MathJax should be used depends on the viewer rather than the generator
> of the HTML output. Some hints on how to detect internet:
> https://stackoverflow.com/questions/189430/detect-that-the-internet-connection-is-offline
>   Or maybe the timeout period can just be reduced?

The timeout I was concerned with is not under the HTML page's control 
anyway. It's browser-specific when trying to resolve the URL ref 
pointing to MathJax. We've not even got to loading the JS file yet when 
we run into this problem.

   David