I have chosen the wrong flight dates because your date picker is broken
Seriously, UI components are hard. Even a lowly button is difficult to get right, where ”right” means ”whatever smart people have converged on after 30 years of GUIs”. Anything more complex than a button is much much harder. There are a lot of things wrong with the whole ”web app” concept but to me maybe the most important is everybody trying to implement their UI widgets from scratch. And I say this as someone whose job used to be writing and maintaining a webapp UI framework.