title: HTML5 <div> centered inside <body>
body: <p>Is there a way to place a div inside body, centered, with given left-right margins equal to x and top-bottom margins, equal to y? Nothing except of the div (and its children) is presented in the document.</p>
  
  <p>UPDATE. I want the following:</p>
  
  <p><img src="https://i.stack.imgur.com/gmmeC.png" alt="enter image description here"></p>
  
  <p>Also, I'd be glad to have a more common solution for the case, when x1 != x2, y1 != y2 (though a solution for my particular case x1==x2, y1==y2 is appreciated).</p>