JSFiddle - React, Tailwind, and code Playground

HTML

<div class="flex-container">
  <div class="item-1">div</div>
  <div class="item-2">w=250px</div>
  <div class="item-3">h=250px</div>
  <div class="item-4">w/h=300px</div>
  <div class="item-5">w=350px</div>
  <div class="item-6">w=350px</div>
</div>

CSS

* {
  box-sizing: border-box;
  font-size: 1.5rem;
}

html {
  background: #b3b3b3;
  padding: 5px;
}

body {
  background: #b3b3b3;
  padding: 5px;
  margin: 0;
}

.flex-container {
  background: white;
  padding: 10px;
  border: 5px solid black;
  height: 800px;
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
  /* align-items: center; */
  justify-content: center;
}

.item-1 {
  background: #ff7300;
  color: white;
  padding: 10px;
  border: 5px solid black;
  margin: 10px;
}

.item-2 {
  background: #ff9640;
  color: white;
  padding: 10px;
  border: 5px solid black;
  margin: 10px;
  width: 250px;
  /* font-size: 1.8rem; */
}

.item-3 {
  background: #ff9640;
  color: white;
  padding: 10px;
  border: 5px solid black;
  margin: 10px;
  height: 250px;
}

.item-4 {
  background: #f5c096;
  color: white;
  padding: 10px;
  border: 5px solid black;
  margin: 10px;
  width: 300px;
  height: 300px;
}

.item-5 {
  background: #d3c0b1;
  color: white;
  padding: 10px;
  border: 5px solid black;
  margin: 10px;
  width: 350px;
}

.item-6 {
  background: #d3c0b1;
  color: white;
  padding: 10px;
  border: 5px solid black;
  margin: 10px;
  width: 350px;
}