Configuring modules for email sending and formatting is a pain and confusing. Most mailing modules rely on Mailsystem module to help configure this.
In D7 Mailsystem 3.x there's an improved method of doing this by using a delegator class. https://www.drupal.org/project/mailsystem/issues/1508104. I propose that the class and related functions be put into core. The UI can stay out of core.
This will help any combo of modules share the responsibility of sending emails. One module can be used for formatting HTML (MimeMail, HTMLmail) and another for sending (SMTP, PHPMailer, Postmark, SendGrid, ...). The popular modules can then use the new class to allow other modules to share in the work and also provide a simple UI (without needing to install Mailsystem if they don't need that extra power).
Recent comments
The specific tags I need are "<ruby>" and "<rt>" Wow, yeah, that's a pain with CKEditor5. A custom plugin, for sure, but no clue, what the code has to look like...
Specific tags to work in CKEditor 5
I think maybe the problem is with CKEditor stripping the tags - it can be more aggressive than what you would expect from the text format settings. I copied your examples into an example page...
Specific tags to work in CKEditor 5
This saved my day, exactly what I had cooked up but I could not find the necessary settings. Thanks a bundle :)
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