When trying to fix the CORS error a few days ago, you made a change in the fetching code so now pages are fetched using fetch instead of fetchProxy.
adaa08a?branch=adaa08ac90758703e6294003b5a24a0e74286e17&diff=unified#diff-0f292e46fc1906eacb3fdbdaced082678d3b8d83fb44ab79b927d2abd836c358L107
Now, when using a page which has cors disabled, like dropbox, the service doesn't work.
http://htmlpreview.github.io/?https://www.dropbox.com/s/mgw65rktedx4pjl/Hello%20world.html?raw=1
Was that change a forgotten invalid fix? I recommend using fetchProxy again, so that dropbox links (and others) works again.