Редактор коду W3Schools - Tryit

<!DOCTYPE html>
<html>
<head>
<style>
.flex-container {
  display: -webkit-flex;
  display: flex;
  -webkit-flex-flow: row wrap;
  flex-flow: row wrap;
  font-weight: bold;
  text-align: center;
}

.flex-container > * {
  padding: 10px;
  flex: 1 100%;
}

.main {
  text-align: left;
  background: cornflowerblue;
}

.header {background: coral;}
.footer {background: lightgreen;}
.aside {background: moccasin;}

@media all and (min-width: 768px) {
  .aside { flex: 1 auto; }
  .main    { flex: 3 0px; }
  .aside { order: 1; }
  .main    { order: 2; }
  .footer  { order: 4; }
}
</style>
</head>
<body>

<div class="flex-container">
  <header class="header">Header</header>
  <aside class="aside">Aside</aside>
  <article class="main">
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Quisque sed ex turpis. Cras luctus nibh lectus, in ullamcorper ex tempor eleifend. Nulla bibendum, eros a consequat vestibulum, orci massa fermentum quam, sed commodo nunc ex vitae nisl. Aliquam ullamcorper interdum est nec tincidunt.</p>
  </article>
  <footer class="footer">Footer</footer>
</div>

</body>
</html>

​x
 
<!DOCTYPE html><html><head><style>.flex-container {  display: -webkit-flex;  display: flex;  -webkit-flex-flow: row wrap;  flex-flow: row wrap;  font-weight: bold;  text-align: center;}​.flex-container > * {  padding: 10px;  flex: 1 100%;}​.main {  text-align: left;  background: cornflowerblue;}​.header {background: coral;}.footer {background: lightgreen;}.aside {background: moccasin;}​@media all and (min-width: 768px) {  .aside { flex: 1 auto; }  .main    { flex: 3 0px; }  .aside { order: 1; }  .main    { order: 2; }  .footer  { order: 4; }}</style></head><body>​<div class="flex-container">  <header class="header">Header</header>  <aside class="aside">Aside</aside>  <article class="main">    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Quisque sed ex turpis. Cras luctus nibh lectus, in ullamcorper ex tempor eleifend. Nulla bibendum, eros a consequat vestibulum, orci massa fermentum quam, sed commodo nunc ex vitae nisl. Aliquam ullamcorper interdum est nec tincidunt.</p>  </article>  <footer class="footer">Footer</footer></div>​</body></html>                