Steps to reproduce:

1. Enable source=true for a richTextField
2. Open this richTextField
3. You can see that Strikethrough is not available in the toolbar.
4. Switch to the Source mode
5. Insert <s>something</s>
6. Switch back from Source mode

Expected result

I can see the text with strikethrough.

Actual result

The s tag is removed by CKEditor because Strikethrough is not allowed from the toolbar.

Workaround

1. By hotfixing the default magnolia config (Magnolia OnPrem) or providing a custom config (Cloud) from your light module:
2. Replace "Strikethrough" with "Strike"
3. Replace source: false, with source: true,

4.       $type: richTextField
         configJsFile: http://localhost:8080/.resources/ckeditor/config-magnolia.js
         validators: {} # Only if you trust your editors!
    

1. Clear your browser cache.
2. Open the dialog with richTextField again.
3. You might need to adjust other settings as now the config is loaded only from this file instead of from the richTextField properties.