Mach mal - HTML, CSS und JavaScript ausprobieren!

<!DOCTYPE html>
<html lang="de">
  <head>
    <meta charset="utf-8">
    <title>JSL142 Window Methoden</title>
  </head>
  <body>
    <button onclick="fensterAuf()">Neues Fenster öffnen</button>
    <button onclick="fensterMove()">Bewege das Fenster</button>
    <button onclick="fensterSize()">Resize window</button>
    <script>
      var fenster;
      function fensterAuf() {
        fenster = window.open("", "Info",
                              "width=200, height=100, top=200");
        fenster.document.write("<p>Wichtige Infos</p>");
      }

function fensterMove() {
        fenster.moveBy(10, -10);
        fenster.focus();
      }

function fensterSize() {
        fenster.resizeTo(450, 650);
        fenster.focus();
      }
    </script>

<hr>

<div id="rotesDiv" style="height: 50px; background: red;"></div>
    <script>
      var element = document.getElementById("rotesDiv");
      var dieCSSeigenschaft = window.getComputedStyle(element);
      var ausgabe = dieCSSeigenschaft.getPropertyValue("width");
      alert("Breite in Pixel: " + ausgabe);
    </script>
  </body>
</html>