/* Selects the content on the first page while printing */
@page :first {
  margin-left: 50%;
  margin-top: 50%;
}

:first

<p>First Page.</p>
<p>Second Page.</p>
<button>Print!</button>

@page :first {
  margin-left: 50%;
  margin-top: 50%;
}

p {
  page-break-after: always;
}

document.querySelector("button").onclick = function(){
  window.print();
}