I am experimenting with the blog page header content. Help for imput field says “HTML and shortcodes are allowed”. Then, following code should work, shouldn’t it?
I think your link has some wrong quotation marks. Just tried this and it works: <iframe width="560" height="315" src="https://www.youtube.com/embed/5dezYhcir0g" frameborder="0" allowfullscreen></iframe>
The code with a questionmark was copied from YouTube share window. Anyway, as soon as I paste your code into customizer nothing happens. When I hit publish and close customizer and revert back the whole iframe tage is gone. Other HTML works just fine.
Offtopic: Leo, just have overlooked your comment about skiing π Thank you. The same video with soundrack is here (YouTube have stripped sound) http://www.dailymotion.com/video/x5rxk6i and ofcourse our web http://telezjetele.cz is all about skiing. Assuming from your gravatar you may be ski adicted as well π