Does Plain support HTML email signatures?
No. Email signatures in Plain support plain text and basic markdown only. If you add HTML tags to your signature, they will appear as raw code in the recipient's inbox. You can edit your signature in Settings, under Account.
Using a snippet instead
If you want more formatting than a signature allows, create a snippet with the content you need and insert it into your replies. Go to Settings, then Snippets, and create a new snippet. In the composer, type [ to insert it.
Snippets support markdown and variables such as {{ user.fullName }} and {{ user.publicName }}. You can use them in replies on any channel, including email, Slack and chat. If you have several signature formats, use groups to keep them organized.