Sending HTML mail is not possible out of the box with core and simplenews.
But it's a key requirement for many of our users.
We should define a recommended / best practice setup. Current proposal is swiftmailer (plus of course mailsystem). The following swiftmailer patches are needed:
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
About creating newsletters with HTML, I ran into an edge case making with CKEditor, where image labels were showing unexpectedly, so I added a bit on the doc page https://www.drupal.org/docs/contributed-modules/simplenews/module-overvi... → . I am not sure if it deserves its own issue?