TempEdit On your mark.

TempEdit edits ACP templates. ACP templates are text snippets that can be inserted into text fields and text views. All ACP applications can benefit, but participation is on an opt-in basis. Currently three applications are set to use templates: Outbox, PlistEdit, and Rixedit.

There's no longer any need to learn new syntax to write HTML files - or any other syntax for that matter.

See Also
ACP Users: Lots More Ways to Leave Your Lover