Below are some helpful tips for supported elements and best practices for using them:
You can use the div element within the editor. We recommend you to do this when you want to style a part of page/element with custom CSS.
Span elements are only saved when you at least add an attribute to the element. Spans without attributes have no use and are therefore removed to keep the code clean.
Paragraph elements are not allowed and will be automatically converted to line breaks.
Style attributes and scripts elements are forbidden and will be stripped. You can use the Custom CSS and Custom JS sections in the Design section of your Admin area to utilize these.
You can find other articles about front-end customization here.