<div id="container1" class="container">
  <p>1.</p>
  <p>width: 100%;<br />
  max-width: 800px;</p>
</div>

<div id="container2" class="container">
  <p>2.</p>
  <p>width: 800px;<br />
    max-width: 100%;</p>
</div>

@import url(https://fonts.googleapis.com/css2?family=Architects+Daughter&display=swap);

:root {
  --pink: #E726B5;
  --yellow: #FFE699;
  --cream: #FBE5D6;
  --green: #70AD47;
  --red: #FF0000;
  
  --text-shadow: 0.3px 0.3px #000000e0;
}

* {
  text-shadow: var(--text-shadow);
  font-size: 20px;
  font-family: 'Architects Daughter';
  box-sizing: border-box;
}

body {
  height: 100vh;
  background-color: var(--yellow);
  padding-top: 10px;
}

.container {
  margin: 0 auto 16px; /* Center */
  padding: 18px;
  font-size: 24px;
  border: 3px dashed var(--pink);
  display: flex; 
  gap: 30px;
}

#container1 {
  width: 100%;
  max-width: 800px;
  background-color: Salmon;
}

#container2 {
  width: 800px;
  max-width: 100%;
  background-color: var(--cream);
}

p {
  margin: 0;
}