Just thought I'd add a comment to share my recent experience. I had an iframe tag for embedding a video that is a countdown time to an upcoming event, and thus having the video play automatically is desirable.
Dropping the tag into a code block was not ideal, as the code block was not responsive to browser window changes. I was placing two videos side-by-side, so having them resize was important. One would resize, and the other would not.
I ended up discovering that I could use a video block while also dropping in the iframe tag I had been given. I clicked the gear icon to the right of the URL box. That opened an Edit Embedded Data dialog. I took a shot and pasted the iframe tag into that box, and that did the trick. Despite the iframe tag including a width and a height, the video resizes along with the browser screen, and I'm able to have my two videos side-by-side. One is a countdown timer that auto starts. The other is a video that the user must click to play.
