# open Admin Console
# go to {nav Configuration > Website > Advanced} section
# set `Editing Window Style` to `Modal Window`
# go to {nav Website & Content > Browse Website} section
# select `Content Mode` (in top frame)
# input iframe tag like `<iframe allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/seI3EA7zafw" width="560"></iframe>` into some CMS block and press `Save` button
# confirm that no modal window closes