Edit in JSFiddle

var key = "draft",
    contentEl = document.getElementById("content");

// load from save
contentEl.innerHTML = localStorage[key] || '';

// save on change
contentEl.onkeyup=function() {
    localStorage[key] = contentEl.innerHTML;
};

<div id="content" contenteditable="true">
</div>

body {
    margin: 0;
    padding: 0;
}

#content {
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
}