After the longest time I switched to the default Hugo markdown rendered, Goldmark, from Blackfriday. I delayed the switch for the longest time but in version 0.100 they removed the old markdown rendered that was deprecated.

Unfortunately this meant all my embedded HTML in markdown stopped working.

I solved this by enabling this option in my config.yaml:

markup:
  goldmark:
    renderer:
      unsafe: true

or, in config.toml:

[markup.goldmark.renderer]
  unsafe = true