W3Schools Tryit Editor

<!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;}
.aside1 {background: moccasin;}
.aside2 {background: violet;}

@media screen and (min-width: 600px) {
  .aside { flex: 1 0px; }
}

@media screen and (min-width: 800px) {
  .main  { flex: 3 0px; }
  .aside1 { order: 1; } 
  .main  { order: 2; }
  .aside2 { order: 3; }
  .footer  { order: 4; }
}
</style>
</head>
<body>

<div class="flex-container">
  <header class="header">Header</header>
  
  <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>
  
  <aside class="aside aside1">Aside 1</aside>
  <aside class="aside aside2">Aside 2</aside>
  
  <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;}.aside1 {background: moccasin;}.aside2 {background: violet;}​@media screen and (min-width: 600px) {  .aside { flex: 1 0px; }}​@media screen and (min-width: 800px) {  .main  { flex: 3 0px; }  .aside1 { order: 1; }   .main  { order: 2; }  .aside2 { order: 3; }  .footer  { order: 4; }}</style></head><body>​<div class="flex-container">  <header class="header">Header</header>    <article class="main">