As of MDL-62491, moodle_content_writer.php loads bootstrap's CSS file from a CDN.

This is fragile for several reasons:

• It requests the URL with a plain fopen(), without using the HTTP proxy if one is configured.
• Failure of the request is not handled, so the process fails with an error if the CDN cannot be contacted.
• The code is called from within unit tests, so the tests will fail if the CDN cannot be contacted.  I think its better to avoid connecting to external services in a unit test.

I propose that we have a local copy of the CSS files in the codebase, and copy the appropriate one into the export rather than downloading it from the CDN.