difference between height and background cover code example

Example 1: css background image cover center

html { 
  background: url(images/bg.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

Example 2: html background size fit text

<div class="myDiv">
    <div class="bgImage">
        <h1>My Image</h1> 
    </div>
</div>

<style>
.myDiv {
	text-align: center;
}
.myDiv h1 {
  display: inline-block;
  font-size: 3.5em;
  border-radius: 20px;
  background: rgba(43, 166, 203, 0.5); /* alpha value for background */
  padding: 0.5em 0.2em 0.8em 0.5em;
}
</style>