how to make a button centered in html5

Code Example 1

<!--Put the button inside a p like this-->
<p style="text-align: center;"><button>See?</button>Whatever</p>

Code Example 2

.center {
  display: flex;
  justify-content: center;
  
  align-items: center;
  height: 200px;
  border: 3px solid 
  green; 
}