Having installed SSL on my forum a few months ago, I decided to look into HSTS last night thanks to this thread, and finally created an .htaccess file that worked ( not as easy as it sounds ).
Just now I saw the padlock still had insecure content, and via Chrome inspect element found it was simply one link on the index.php. When I took out the google search plugin, the insecure bit had vanished. I mention this because of the irony that old Google which has been shoving SSL on all of us, apparently can't just resolve http:// links to Google into https:// links automagically...
.
So I'm glad for this thread: mine was the cheapest about $4 a year from gogetsll ; but multi-domain costs about $15 a year. It's getting to be a necessary component of websiting now. I think it would be best for Danbooru...