<!DOCTYPE html>
<html>
<meta name="viewport" content="width=device-width">
<style>
body {
    width: 100%;
    height: 100%;
    margin: 0;
    position: absolute;
}
#container {
    width: calc(100% - 100px);
    height: calc(100% - 100px);
    margin: 50px;
}
#editor {
    width: 100%;
    height: 100%;
}
</style>
<div id="container"><iframe id="editor" src="data:text/html,<body%20contenteditable%20style='width:%20100%;%20height:%20100%;%20margin:%200;'>"></iframe></div>
</html>