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HTML

              
                <header>
  <h1>CSS is awesome!</h1>
</header>

<section>
  <h2>Welcome to the dark side of the CSS</h2>
  <p>Ширина body здесь всего 400px (подсвечена рамкой при наведении).</p>
  <p>А фон каждого раздела растянут до краев окна.</p>
  <p>И никакого горизонтального скроллинга, как могло бы быть с псевдоэлементами.</p>
</section>

<footer>
  <p>body is only 400px wide!</p>
  <p><a href="https://commons.wikimedia.org/wiki/File:MarsPanoramaa.jpg">Image</a> from Wikimedia</p>
</footer>
              
            
!

CSS

              
                header {
  border-image: radial-gradient(100% 100% at 80% 0%, #003 98.6%, gold 99% 99.6%, transparent 100%) 0 49.99% fill / 0 55vw / 2em calc(55vw - 200px);
}

section {
  margin: 20vw 0;
  border-image: conic-gradient(from 265deg at 135%, transparent, blue 0.1deg 9.9deg, transparent 10deg) 49.99% fill / 44vw 55vw / 40vw calc(55vw - 200px);
}

footer {
  border-image: url(https://upload.wikimedia.org/wikipedia/commons/thumb/f/f9/MarsPanoramaa.jpg/1280px-MarsPanoramaa.jpg) 49.99% fill / 50% 55vw / 10vw calc(55vw - 200px);
  --mask: linear-gradient(-5deg, blue 60%, transparent 60.3%) 100% 49.9% fill / 150% 50% 0/ 10vw 55vw 1em;
  -webkit-mask-box-image: var(--mask);
  mask-border: var(--mask);
  font-size: 50%;
}

body {
  width: 400px;
  margin: auto;
  font: 1.5em / 1.5 sans-serif;
  color: #fff;
  background: conic-gradient(at 1px 19px,#000 25%, #333 0) 0 0/20px 20px
}

body:hover {
  outline: 2px dashed red;
}

a {
  color: gold;
}
              
            
!

JS

              
                
              
            
!
999px

Console