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steemit.com does not support embedding of external content (likely for security reasons, as your private keys are a target to hackers).

Great stuff!

Why does embedding not work with the video tag? Is it because you're using MediaSource extensions or something? Why does youtube/vimeo embedding work ok?

I dunno but @furion says that the videos are embeddable, but steemit.com has to whitelist Viewly first.

So is there any criteria for whitelisting a website on Steemit?

That sounds positive, and should be pretty straightforward if so then!

Let's hope for the best and may it happen sooner than later!

Steemit support YouTube and Soundcloud iframes. It does not support Mixcloud I found out. As well as it doesn't support Viewly :( Didn't try other external content services so far.

viewly is pretty new although it's been in the works for quite a while, I'm wondering why a update isn't just patched in for it.

A big question for me as well, since it cannot be simply 3rd party content not enabled through iframes because of security, since then YouTube and Soundcloud would also need to be disabled.

they are probably whitelisted and integrated. It's a matter of web-design back end probably the Steemit Inc. team has the site :)