index.html
<!DOCTYPE html>
<html>
  <head>
    <link rel="stylesheet" href="style.css">
  </head>
  <body>
    <main></main>
    <script src="p5.min.js"></script>
    <script src="sketch.js"></script>
  </body>
</html>
Preview
sketch.js
function setup() {
  createCanvas(300, 300);
  background('#000');
}
style.css
body {
  margin: 0;
  padding: 0;
}

main {
  display: flex;
  align-items: center;
  justify-content: center;
  min-width: 100vw;
  min-height: 100vh;
}