Thanks jlaps,

So a bit of checking and this isn't SN's HTMLSanitizer that's stripping your formatting but TinyMCE v5 itself (which SN uses) that strips out invalid HTML. Looking at their documentation, it doesn't look possible to disable this behaviour https://www.tiny.cloud/docs/tinymce/latest/content-filtering/

As a validation step, I added tinymce_allow_all=true to the html field but it resulted in the below behaviour

Using one of your mail scripts as an example (${mail_script:element.inline_style.generator.tr}) is it returning more than one attribute key and value? Reason being, if you did the following, it's not stripped as it's considered valid HTML