<iframe src="mypage.html" style="position:fixed; top:0px; left:0px; bottom:0px; right:0px; width:100%; height:100%; border:none; margin:0; padding:0; overflow:hidden; z-index:999999;"> Your browser doesn't support iframes </iframe>

You can also use viewport-percentage lengths to achieve this. The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. Where 100vh represents the height of the viewport, and likewise 100vw represents the width.

body {
    margin: 0;            /* Reset default margin */
}
iframe {
    display: block;       /* iframes are inline by default */
    background: #000;
    border: none;         /* Reset default border */
    height: 100vh;        /* Viewport-relative units */
    width: 100vw;
}

<iframe></iframe>