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use fas in the class.
<i class="far fa-exclamation-triangle"></i> --> NO
<i class="fas fa-exclamation-triangle"></i> --> YES
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<link href="//maxcdn.bootstrapcdn.com/font-awesome/4.1.0/css/font-awesome.min.css" rel="stylesheet">
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Make sure you don't keep the name of all the font families by using this (*). If yes, you can remove the font-family property.
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<!-- Wrong -->
<i class="fa-search"></i>
<!-- Correct -->
<i class="fa fa-search"></i>