JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="A"> A </div>
  <div class="B"> B </div>
</div>

CSS

.container {
  display : flex;
}
.A {
  flex : 1;
}
.B {
  flex : 1;
}
@media only screen and (max-width: 700px) {
 .container {
   flex-direction : column;
 }
 .A {
      order : 2;
  }
 .B {
      order : 1;
    }
}