image isnt centering html code example

Example 1: html center image

/* It's better practice to do this in CSS, not HTML. 
So here is the CSS code! */
img {
  display: block;
  margin-left: auto;
  margin-right: auto;
}

Example 2: center image html

.centered {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  -webkit-transform: translate(-50%, -50%);
  -moz-transform: translate(-50%, -50%);
  -o-transform: translate(-50%, -50%);
  -ms-transform: translate(-50%, -50%);
  
  font-size: 20px;
  background-color: cyan;
  border: darkgreen 5px solid;
  padding: 5px;
  z-index: 100;
}