For responsive web design and for all projects in freecodecamp will not work only the challenges will work and I think freecodecamp will check your projects or else you have a run button the codepen so that it checks your code wheater that’s correct or not and if you wanna want to know is my project correct so simply message in the forum so that the leaders can check and give suggestions
And here’s an image of how to run that in codepen.io
Happy coding