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HTML 

              
                <html lang="es">
<head>
  <meta charset="UTF-8">
  <title>HTML5 básico con FlexBox</title>
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>

  <main>
    <header><h1>Cabecera (header)</h1></header>    
    <nav>
      <div class="menu">ítem 1</div>
      <div class="menu">ítem 2</div>
      <div class="menu">ítem 3</div>
    </nav>
    <section>
      <article><h2>Artículo 1</h2>
      Lorem ipsum dolor sit amet, consectetur adipisicing elit. Sit obcaecati quidem et temporibus sequi nesciunt neque, odio earum, provident architecto voluptate fuga quod ut vero asperiores magnam excepturi nihil molestiae.</article>
      <article><h2>Artículo 2</h2>
      In autem molestiae, voluptatem ratione omnis rerum dolorem deleniti porro? Repudiandae, pariatur dolorum doloribus a quibusdam, autem porro dignissimos quaerat illum magnam, at perspiciatis! Quasi quas praesentium nisi, nesciunt officia.</article>
      <article><h2>Artículo 3</h2>
      Dignissimos expedita necessitatibus eius! Suscipit quo delectus ratione possimus sit nihil distinctio saepe expedita rerum voluptates dolorem adipisci atque accusamus quisquam commodi ut, eligendi aliquam molestiae nobis ex? Aliquam, consequatur!</article>
      <article><h2>Artículo 4</h2>
      Dignissimos expedita necessitatibus eius! Suscipit quo delectus ratione possimus sit nihil distinctio saepe expedita rerum voluptates dolorem adipisci atque accusamus quisquam commodi ut, eligendi aliquam molestiae nobis ex? Aliquam, consequatur!</article>
    </section>
    <aside>
      <h3>Barra lateral (aside)</h3>
      <p>Voluptatibus minima quod tenetur fugiat atque nisi ex necessitatibus est. Incidunt facilis quis voluptate repellat eveniet, iure quae! Optio, hic!</p>
    </aside>
    <footer><h5>Pie de página (footer)</h5></footer>

  </main>
</body>
</html>
!

CSS 

              
                /*  ===================================
      Responsive Design, Mobile-First
      (cc) 3con14.pro
    =================================== */

/* Reglas comunes ------------------------------- */
* {
  margin     : 0;
  padding    : 0;
  box-sizing : border-box;
  transition : all 0.5s ease-out;
}
body {
  font       : 16px sans-serif;
  background : rgba(0,0,128,0.2);
  padding    : 1rem;
}
main,header,nav,section,article,aside,footer {
  outline    : 1px solid #fff;
  background : rgba(0,0,0,0.1);
}
header,nav,article,aside,footer {
  padding: 1rem;
}
section { background: rgba(255,0,0,0.1); }
aside   { background: rgba(255,255,0,0.1); }


/* Smartphone ----------------------------------- */
main {
  display        : flex;
  flex-direction : row;
  flex-wrap      : wrap;
}
nav {
  display        : flex;
  flex-direction : column;
}
.menu {
  background : rgba(0,0,255,0.2);
  margin     : 1px;
  padding    : 2px 1em;
  text-align : center;
}
header,nav,section,aside,footer {
  width: 100%;
}


/* Tablet --------------------------------------- */
@media all and (min-width: 568px) {
  section { flex: 3; order: 2; }
  aside { flex: 1; order: 1; }
  footer { order: 3; }
  nav { 
    flex-direction  : row;
    justify-content : flex-end;
  }
}
/* Desktop -------------------------------------- */
@media all and (min-width: 768px) {
  section {
    display   : flex;
    flex-wrap : wrap;
    order: 1;
  }
  article { width: 50%; }
  nav     { justify-content: center; }
}
!

JS
!
999px

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