[html] Set div height to fit to the browser using CSS

I think the fastest way is to use grid system with fractions. So your container have 100vw, which is 100% of the window width and 100vh which is 100% of the window height.

Using fractions or 'fr' you can choose the width you like. the sum of the fractions equals to 100%, in this example 4fr. So the first part will be 1fr (25%) and the seconf is 3fr (75%)

More about fr units here.

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.container{
  width: 100vw;
  height:100vh; 
  display: grid;
  grid-template-columns: 1fr 3fr;
}

/*You don't need this*/
.div1{
  background-color: yellow;
}

.div2{
  background-color: red;
}
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<div class='container'>
  <div class='div1'>This is div 1</div>
  <div class='div2'>This is div 2</div>
</div>
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