У вашего броузера проблема в совместимости с HTML5
This Flexbox tutorial walks you through how to build out a responsive navigation bar using tools such as Flexbox, HTML, CSS media queries, and JavaScript. The final code includes a navigation bar that hides on smartphones, becoming a dropdown, and expands on tablets and desktop screens. Source code and show notes here: https://rails.devcamp.com/flexbox/flexbox-course-capstone-project/flexbox-capstone-project-solution-build-responsive-navbar