So, you've set up your website to embed a YouTube video, and then you realise that it draws on top of your navbar or modals. Changing the z-index doesn't seem to do anything and you're stuck! Luckily, it's easy to fix:
You only need to add a bit of code to the end of your embedding URL:
?wmode=transparent
The full URL will look something like this:
https://www.youtube.com/embed/dQw4w9WgXcQ?wmode=transparent
This will mean YouTube videos respect z-index and don't sit on top of all the other content on your page.