If a page is served over SSL (https), then any content loaded from non-SSL
sources (e.g. http://youtube.com/) will cause errors to show in the
browser. To prevent this, it's best to use the SSL equivelants where they
exist (e.g. https://youtube.com/).
Unfortunately, it isn't possible to accurately determine whether the
current page is loaded over an SSL connection or not in Moodle.
Since including content from an external SSL site on a non-SSL moodle page
does not lead to browser warnings, but non-SSL external content on an SSL
moodle page does, we always use SSL where available.
Note: This does not lead to any additional processing requirements for the
moodle site.