The answer is very simple.

Because email content is not a Post content. it should be secure and clean. This will help emails to pass more antispam systems of Email servers.

The default WordPress emails doesn't have HTML tags, we added a few secure tags (formatting tags and link).

OK thanks.
And there is really no way to add the style tag?

I'm sorry but there is no way to do this in current version. We'll add some filter hooks in next version to allow add new HTML tags. We're planning to release next version in the first half of next month.

ok, thanks for the support  @robert 👍 

Hello,

The tag <IMG /> isn't allow too ?

I try to add this on the header (of the boby ^^ ) of a email but after Save it was deleted :

<img src="https://www.acdc62110.ovh/wp-content/uploads/2018/08/enteteemailACDC-600.png" width="602" height="142" />
<br>
Bonjour [member_name]!<br>