Responsive Intro page

if you are using class to set the display properties none and then change it block or flex when clicked on the submenu may be thats the reason if you can make it look smooth and can't add animation because when you set display none you are literally deleting the html element and you can't animate that . so if you wanna animate like you want to submenu to slide down from top or something then what you can do is don't change the display properties instead set the position absolute and change the top value such that it goes up outside of the page hence it won't show up and when the user click on the button change the top properties to 0 or something that makes it slides down. same with arrow you can transform rotate to rotate the arrow like rotate it 180deg or 90 deg when the button is clicked. i hope it helps you. best of luck