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                <div id="app" class="wrapper">    
  <div class="header">
  <div class="main">
    <div class="sidebar">
      <div class="subheader">
        Subheader of sidebar    
      <div class="sidebar-content">
         Sidebar list      
    <div class="content">
      <div class="subheader">Subheader of content block</div>
      <div class="content-region">
         Main content      
  <div class="pusher"></div>

<div class="footer">


  display: flex /*specify wrapper as the flex container, makes its children flex items and stretch them to fill all the space.*/
  flex-direction: column /*sets direction of flex item in container: lays them out in vertical columns since we have .main and .header blocks as direct childs*/

  flex: 1 /*the shorthand for flex-grow, flex-shrink and flex-basis, the same as flex: 1 1 auto (It can grow, shrink and has default size auto). The flex item fills all available space while by default it will be stretched to fill the container.*/
  display: flex /*specify flex context for .sidebar and .content blocks*/

  width: 300px /*We can leave .sidebar as it is since it is already flex item with default properties (flex: 0 1 auto - can't grow, can shrink) and it's default width:300px is defined already*/ 

.sidebar, .content
  display: flex /*sets flex context for the next elements*/
  flex-direction: column

  flex: 1