Finally got a reply from IJ support

[tanithryudo]

It only took about a year to get back to me, lol.

In any case, so the issue with embedded youtube not showing up on crossposts is due to the different syntax required.

DW uses the iframe and /iframe tag (which is what youtube also uses when you share). But IJ uses the embed /embed tag. Once I replace iframe with embed, the video shows up on IJ.

EDIT: looks like DW can use both iframe and embed, while IJ can only use embed tag. Good to know.

Test iframe

Watch on YouTube

Test embed

Watch on YouTube

Comments

[tanithryudo]

At first I thought it was youtube changing their default code when sharing videos. But then I checked my early history of embedded video posts which I remember used to work, and those were also using iframe. So it looks like it was a change on the IJ side which specifically removed support for iframe. :/

[cashew]

Blech. I don't know why they update the code like that. Is it for security purposes? I recall something about worrying that iframe can allow script injections in blog posts. That was like over 10 years ago, tho'.