Jen Simmons is an American graphic designer, web developer, educator and speaker known for her expertise in web standards, particularly HTML and CSS. She is a member of the CSS Working Group and has been prominent in the deployment of CSS grid layout. She worked as a developer advocate at Mozilla and later at Apple.

Cascading Style Sheets (CSS) is a style sheet language used for specifying the presentation and styling of a document written in a markup language such as HTML or XML. CSS is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript.