About half of Notion's supported embed types are generic iframes. The rest either have edge cases, custom formatting, or use a third-party script to embed content.
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https://twitter.com/emichan_iguana/status/1275210350665719810?s=20
https://open.spotify.com/playlist/2MgkBl2rQnPdF3WZ6ciajc?si=YO7W88RIQbCdeQpEPsq0eg
https://open.spotify.com/episode/1sTz31srankwiOvIhhjgA9
https://open.spotify.com/episode/0Ok2HlpJ06C8c2Wrhm8Nkv