Reponsive QR Code Design

Some recommendations regarding your code that could be of interest to you.

HTML

In order to fix the accessibility issues:

• You need to replace <div class="container"> with the <main class="container"> tag. You'd better use Semantic HTML, and you can also reach more information about it from Using Semantic HTML Tags Correctly.
• Each main content needs to start with an h1 element. Your accessibility report states page should contain a level-one heading. So, you need to use a <h1> element in the <main> tag instead of using <h2>. You can replace your <h2>Improve your front-end skills by building projects</h2> element with the <h1>Improve your front-end skills by building projects</h1> element.

CSS

• If you want to center it easily, you can use flexbox in the body and update it in this way:

body {
  background-color: hsl(212, 45%, 89%);
  font-family: "Outfit", sans-serif;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
}

• then, you don't need to give margin to .container . This .container is the updated version:

.container {
   align-items: center;
   max-width: 300px;  // max-width makes it responsive
   background: #fff;
   padding: 10px; 
   border-radius: 5%;
}

Hope I am helpful. :)