You can use HTML in 3 different ways when designing a specific Application type or Email Template.
1. Editor
Note: HTML editor takes only the content in <body></body> tags. The styles can be assigned via inline CSS or attached external files.
2. Advanced HTML: this option allows you to create HTML code with internal CSS. Unlike HTML Editor, Advanced HTML allows you to see the entire HTML code.
3. Upload File: The Upload File button on the Basics tab of the Create Application page allows you to upload external CSS and JS files to use with the HTML code.
Related Resources:
Create a Registration Application
Create a Landing Page Application
Create an Email Template
Use Dynamic Tags