Hello @NoahL43 ,
While you could potentially use older HTML coding, several elements were either deprecated or reworded with the advent of HTML5, so we definitely recommend using HTML5 for building your emails in the custom code editor. I'd definitely recommend following along with some of these articles when building your first custom code email, to make sure you're getting it setup as efficiently as possible while avoiding common errors:
Create an email with custom HTML code
Errors in a custom code email (also covers unsupported tags and illegal characters)
... View more
