one hundred percent width

HTML

<div class="example" style="width: 100%">
  <p>Image in 100% wide <code>div</code> tag</p>
  <img src="http://via.placeholder.com/350x150"/>
</div>

<div class="example" >
  <p>100% wide <code>img</code> tag</p>
  
  <img style="width: 100%" src="http://via.placeholder.com/350x150"/>
</div>

CSS

.example {
  border: 1px solid black;
  margin: 1em 0em;
}