Center iframe

Edit: FLEX solution

Using display: flex on the <div>

div {
    display: flex;
    align-items: center;
    justify-content: center;
}

JSFiddle: https://jsfiddle.net/h9gTm/867/

One solution is:

div {
  text-align:center;
  width:100%;
}
iframe{
  width: 200px;
}

<div>
  <iframe></iframe>
</div>

 Run code snippetExpand snippet

JSFiddle: https://jsfiddle.net/h9gTm/

edit: vertical align added

css:

div {
    text-align: center;
    width: 100%;
    vertical-align: middle;
    height: 100%;
    display: table-cell;
}
.iframe{
  width: 200px;
}
div,
body,
html {
    height: 100%;
    width: 100%;
}
body{
    display: table;
}

JSFiddle: https://jsfiddle.net/h9gTm/1/