I've just checked with the SSL Server Test if I implemented the SSL certificate on my server correctly. I got a grade A in their ranking but you can get an A+ if you have activated HSTS. After some seaching I found out that Google might treat HSTS as a ranking factor. So it seems to be relevant in terms of SEO. Before implementing HSTS I've got some questions.
At the moment (without HSTS) I get a "mixed content" warning if resources are loaded via http. Does "mixed content" actually exists if HSTS is activated?