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<div class="wrapper">
  <div class="content">
    <p>Main Content</p>
    <p>Space, the final frontier. These are the voyages of the Starship Enterprise. Its five-year mission: to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before.</p>
  </div>
  <div class="side">
    <p>Sidebar</p>
    <p>Space, the final frontier. These are the voyages of the Starship Enterprise. Its five-year mission: to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before.</p>
  </div>
</div>
<div class="footer">
  <p>Footer</p>
</div>
* {
  box-sizing: border-box;
  -moz-box-sizing: border-box; /* Firefox */
}
.wrapper div {
  padding: 10px;
}
.content {
  background: #add;
}
.side {
  background: #aad;
}
.footer {
  width: 100%;
  height: 50px;
  background: #dad;
  padding: 10px;
}
@media screen and (min-width: 600px) {
  .wrapper {
    margin: auto;
    display: table;
    direction: rtl;
    max-width: 800px;
  }
  .wrapper div {
    direction: ltr;
  }
  .content {
    display: table-cell;
    max-width: 600px;
    background: #add;
  }
  .side {
    display: table-cell;
    width: 200px;
    background: #aad;
  }
  .footer {
    margin: auto;
    max-width: 800px;
  }
}