This is related to #475. I see the point of using protocol relative URL's, but I think you have to check if the corresponding https links work before converting all http links, e.g. this does not exist: https://d3js.org/d3.v2.min.js
Good point. One solution to this problem would be to use CDNs to link to all resources, since there is a higher chance that they serve both http and https. Thanks for bringing this up.
[ ] use cdns for all js/css links to ensure that both http and https links work (thanks @yihui)
This is related to #475. I see the point of using protocol relative URL's, but I think you have to check if the corresponding https links work before converting all http links, e.g. this does not exist: https://d3js.org/d3.v2.min.js