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CSS

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HTML 

              
                <main>
  <aside class="smDown">
    <h2>Search for articles</h2>
    <form>
      <label class="visually-hidden" for="search">Search</label>
      <input id="search" name="search" />
      <button type="submit">Search</button>
    </form>
  </aside>
  <article>
    <h1>Lorem ipsum dolor</h1>
    <h2>Sit amet</h2>
    <p>
       Lorem ipsum dolor sit amet, consectetur adipiscing elit. Quisque magna orci, egestas vel gravida ut, fringilla quis tortor. Morbi elementum nulla et urna feugiat, nec mollis arcu convallis. Suspendisse eu purus iaculis, laoreet purus commodo, hendrerit quam. Sed vel mauris ullamcorper, dapibus tellus in, volutpat elit. Morbi tincidunt massa eget nisi suscipit ultrices eu ac quam. Sed elementum, felis sit amet volutpat semper, purus purus malesuada nibh, in egestas nisi orci a odio. Phasellus velit massa, vulputate vitae ante nec, congue porta risus.
    </p>
    <button>Register now</button>
    <h2>Massa nulla</h2>
    <p>
      Aenean tristique massa nulla, a accumsan velit ullamcorper et. Sed ut eleifend neque. Donec eget accumsan ipsum. Vestibulum orci massa, finibus et diam vel, consectetur sollicitudin ante. Nulla eget mauris facilisis, tincidunt mi sit amet, imperdiet eros. Donec ultrices magna nibh, sit amet volutpat eros porta malesuada. Aliquam bibendum, libero nec pellentesque feugiat, urna diam scelerisque sapien, efficitur ultrices ex urna ut mauris. Aliquam eget eleifend tellus, eget dictum augue. Nam volutpat eleifend scelerisque. Maecenas vestibulum leo sit amet massa tempus porttitor. Quisque sed magna ac ipsum molestie mollis. Pellentesque ligula quam, sagittis non dictum in, tempus ac urna. Phasellus in nibh pharetra, dignissim enim faucibus, accumsan libero.
    </p>
    <p>
      Proin auctor <a href="#">felis nec nulla</a> ultrices porta. Integer bibendum eros in leo hendrerit, vel efficitur nibh ornare. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia curae; Vivamus porttitor augue odio. Fusce sollicitudin laoreet tincidunt. Suspendisse ut hendrerit turpis, rhoncus rhoncus odio. Nunc vel cursus lectus. Pellentesque suscipit sit amet nisl luctus faucibus. Donec erat ante, convallis eget dignissim eu, tristique a orci.
    </p>
    <p>
      Suspendisse metus est, lacinia id elit vitae, tincidunt vestibulum eros. Sed porta nulla tortor. Praesent libero est, posuere in fermentum non, feugiat ut nisl. Donec blandit dolor sed justo tincidunt, a pharetra arcu accumsan. Aenean ac arcu vitae neque fermentum vulputate. In pretium ipsum id mattis tincidunt. Mauris quam ex, dignissim ac purus ut, sollicitudin vulputate massa. Proin commodo massa at semper lacinia. 
    </p>
  </article>
  <aside class="mdUp">
    <h2>Search for articles</h2>
    <form>
      <label class="visually-hidden" for="search">Search</label>
      <input id="search" name="search" />
      <button type="submit">Search</button>
    </form>
  </aside>
</main>
!

CSS 

              
                main {
  display: flex;
  flex-direction: column;
  gap: 1.5rem;
}

aside.mdUp {
  display: none;
}

@media (min-width: 50em) {
  main {
    flex-direction: row;
  }
  
  main > article {
    flex-basis: 60%;
  }
  
  aside.smDown {
    display: none;
  }
  
  aside.mdUp {
    display: block;
  }
}

.visually-hidden:not(:focus):not(:active)  {
  clip-path: inset(50%);
  height: 1px;
  overflow: hidden;
  position: absolute;
  white-space: nowrap;
  width: 1px;
}
!

JS
!
999px

Console