Why is flex not working in VS Code or Glitch, but does work in Codepen?

I have a very simple piece of code, and I'm wondering why this (flex) will not work in VS or Glitch, but does in Codepen? How do I make it work in VS and Glitch?

<div class="container"><div class="box-1">Home</div><div class="box-2">About</div><div class="box-3">Services</div><div class="box-4">Contacts</div></div>*{  margin: 0px;  padding: 0px;  box-sizing: border-box;}body{  font-family: sans-serif;  font-size: 25px;  width: 100%;  min-height: 100vh;}.container{  width: 100%;  height: 100vh;  display: flex;}

I tried finding the answer, and using multiple editors.