RE: Myspace: site owners need to turn off features they don't want, all of this can be done from the control panel. Once Tags are installed it is up to the site owner to turn them on, off, or delete them completely. The Custom Tag Editor can be found in the Control Panel at Tools & Information -> Content Rebuilder -> Custom Tag Editor (tab)

RE: Twitter: No video tag currently exists for embedding media from Twitter. Oddly enough their mobile client and app don't even show that they support embedding, only the desktop website ui includes embed codes.

There are some third party services for embedding listed over at UBBDev.

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