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CSS

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HTML 

              
                <h1 class="text-center">Bootstrap 3</h1>
<h2 class="text-center">Resize the window to see the grid that's being used.</h2>
<hr>

<!-- EXTRA SMALL GRID -->
<div class="container text-center">
  <div class="row visible-xs">
    <h3>At this point, the window width is less than 768px <strong></strong>. You'll use Bootstrap's grid for <strong class="bg-color">extra small screens</strong>.</h3>
    <hr>
    <h4>You have to wrap your rows inside a <strong>container</strong> or a <strong>fluid container</strong>.</h4>
    <h4>For this viewport size, the width of the container expands to fill the window's width.</h4>
    <h4>If you select the fluid-container, its width will always have the width of the viewport size.</h4>
    <hr>
    <h4>Each row can have up to 12 columns.</h4>
    <h4>The class prefix you can use to set the size of your columns is <code>col-xs-#</code>.</h4>
    <p>For instance, if you want two equal-sized columns, you'll use the <code>col-xs-6</code> class.</p>
    <p>Keep in mind that in case you'll not specify a column size for extra small screens, they will be appearing stacked. That means, they will occupy the full width of their container (<code>col-xs-12</code>).</p>
    <hr>
    <p>Using <strong>offset</strong> classes (for example, <code>col-xs-offset-2</code>) you can center or move columns to the right. These classes change the <code>margin-left</code> value of each column.</p>
    <p>Using <strong>push & pull</strong> classes (for example, <code>col-xs-push-2</code>) you can move columns to the left and right respectively. These classes change the <code>left</code> and <code>right</code> values of each column.</p>
    <p>Using <strong>visibility</strong> classes (for example, <code>visible-xs</code>) you can adjust the content that will be appearing in this viewport size.</p>
    <hr>
    <p>Demo by George Martsoukos. <a href="http://www.sitepoint.com/grid-system-comparison-bootstrap-vs-foundation" target="_blank">See article</a>.</p>
  </div>
  
  <!-- SMALL GRID -->
  <div class="row text-center visible-sm">
    <h3>At this point, the window width is 768px and up <strong></strong>. You'll use Bootstrap's grid for <strong class="bg-color">small screens</strong>.</h3>
    <h4>You have to wrap your rows inside a <strong>container</strong> or a <strong>fluid container</strong>.</h4>
    <hr>
    <h4>For this viewport size, the width of the container is 750px.</h4>
    <h4>If you select the fluid-container, its width will always have the width of the viewport size.</h4>
    <hr>
    <h4>Each row can have up to 12 columns.</h4>
    <h4>The class prefix you can use to set the size of your columns is <code>col-sm-#</code>.</h4>
    <p>For instance, if you want two equal-sized columns, you'll use the <code>col-sm-6</code> class.</p>
    <p>Keep in mind that in case you'll not specify a column size for small screens, they will inherit the styles from the extra small screens.</p>
    <hr>
    <p>Using <strong>offset</strong> classes (for example, <code>col-sm-offset-2</code>) you can center or move columns to the right. These classes change the <code>margin-left</code> value of each column.</p>
    <p>Using <strong>push & pull</strong> classes (for example, <code>col-sm-push-2</code>) you can move columns to the left and right respectively. These classes change the <code>left</code> and <code>right</code> values of each column.</p>
    <p>Using <strong>visibility</strong> classes (for example, <code>visible-sm</code>) you can adjust the content that will be appearing in this viewport size.</p>
    <hr>
    <p>Demo by George Martsoukos. <a href="http://www.sitepoint.com/grid-system-comparison-bootstrap-vs-foundation" target="_blank">See article</a>.</p>
  </div>
  
  <!-- MEDIUM GRID -->
  <div class="row text-center visible-md">
    <h3>At this point, the window width is 992px and up <strong></strong>. You'll use Bootstrap's grid for <strong class="bg-color">medium screens</strong>.</h3>
    <h4>You have to wrap your rows inside a <strong>container</strong> or a <strong>fluid container</strong>.</h4>
    <hr>
    <h4>For this viewport size, the width of the container is 950px.</h4>
    <h4>If you select the fluid-container, its width will always have the width of the viewport size.</h4>
    <hr>
    <h4>Each row can have up to 12 columns.</h4>
    <h4>The class prefix you can use to set the size of your columns is <code>col-md-#</code>.</h4>
    <p>For instance, if you want two equal-sized columns, you'll use the <code>col-md-6</code> class.</p>
    <p>Keep in mind that in case you'll not specify a column size for medium screens, they will inherit the styles from the previous screens.</p>
    <hr>
    <p>Using <strong>offset</strong> classes (for example, <code>col-md-offset-2</code>) you can center or move columns to the right. These classes change the <code>margin-left</code> value of each column.</p>
    <p>Using <strong>push & pull</strong> classes (for example, <code>col-md-push-2</code>) you can move columns to the left and right respectively. These classes change the <code>left</code> and <code>right</code> values of each column.</p>
    <p>Using <strong>visibility</strong> classes (for example, <code>visible-md</code>) you can adjust the content that will be appearing in this viewport size.</p>
    <hr>
    <p>Demo by George Martsoukos. <a href="http://www.sitepoint.com/grid-system-comparison-bootstrap-vs-foundation" target="_blank">See article</a>.</p>
  </div>
  
  <!-- LARGE GRID -->
  <div class="row text-center visible-lg">
    <h3>At this point, the window width is 1200px and up <strong></strong>. You'll use Bootstrap's grid for <strong class="bg-color">large screens</strong>.</h3>
    <h4>You have to wrap your rows inside a <strong>container</strong> or a <strong>fluid container</strong>.</h4>
    <hr>
    <h4>For this viewport size, the width of the container is 1170px.</h4>
    <h4>If you select the fluid-container, its width will always have the width of the viewport size.</h4>
    <hr>
    <h4>Each row can have up to 12 columns.</h4>
    <h4>The class prefix you can use to set the size of your columns is <code>col-lg-#</code>.</h4>
    <p>For instance, if you want two equal-sized columns, you'll use the <code>col-lg-6</code> class.</p>
    <p>Keep in mind that in case you'll not specify a column size for large screens, they will inherit the styles from the previous screens.</p>
    <hr>
    <p>Using <strong>offset</strong> classes (for example, <code>col-lg-offset-2</code>) you can center or move columns to the right. These classes change the <code>margin-left</code> value of each column.</p>
    <p>Using <strong>push & pull</strong> classes (for example, <code>col-lg-push-2</code>) you can move columns to the left and right respectively. These classes change the <code>left</code> and <code>right</code> values of each column.</p>
    <p>Using <strong>visibility</strong> classes (for example, <code>visible-lg</code>) you can adjust the content that will be appearing in this viewport size.</p>
    <hr>
    <p>Demo by George Martsoukos. <a href="http://www.sitepoint.com/grid-system-comparison-bootstrap-vs-foundation" target="_blank">See article</a>.</p>
  </div>
</div>
!

CSS 

              
                @import url(https://fonts.googleapis.com/css?family=Source+Sans+Pro:400,700);

body {
  font-size: 16px;
}

body, h1, h2, h3, h4 {
  font-family: 'Source Sans Pro', sans-serif;
}

h1, h2, h3, h4 {
  line-height: 1.4;
}

h1 {
  font-weight: 700;
}

p {
  line-height: 1.6;
}

a {
  color: white;
}

h3 {
  margin-bottom: 25px;
}

.bg-color {
  background: #bdbd00;
  padding: 3px;
}

h4, p {
  margin-bottom: 15px;
}

@media only screen and (max-width:767px) {
  body {
    background: lightblue;
  }
}

@media only screen and (min-width: 768px) {
  body {
    background: #4baac8;
  }
}

@media only screen and (min-width: 992px) {
  body { 
    background: LightSteelBlue;
  }
}

@media only screen and (min-width: 1200px) {
  body {
    background: #688ec0;
  } 
}
!

JS 

              
                $(window).on('load resize', function(){
  var $browserWidth = $(window).width();
  $('h3 strong:first-child').text('(' + $browserWidth + 'px)').css('color', 'white');
});
!
999px

Console