MIRA A VER SI ESTO TE SIRVE
<html>
<head>
<script>
function anadir(keyCode){
editor.innerHTML+=(keyCode);
editor.focus;
}
</script>
<style>
#editor{
background-color: #ffffff;
width:80%;
height:200px;
border: 1px solid #666666;
font-family: Palatino Linotype;
font-size: 100%;
text-align:left;
color: #000000;
padding-left:1%;
padding-top:15px;
padding-bottom:15px;
border-style: double;
border-color: #666666;
overflow: scroll;
scrollbar-face-color: #c7c7c7;
scrollbar-highlight-color: #f4f4f4;
scrollbar-shadow-color: #dfdfdf;
scrollbar-3dlight-color: #dfdfdf;
scrollbar-arrow-color: #dfdfdf;
scrollbar-darkshadow-color: #dfdfdf;
border-bottom: 1px double #666666";
}
</style>
</head>
<body>
<div name="content" autofocus contenteditable="true" id="editor">Texto editable</div>
<input value="1" onclick="anadir('botón 1')" type="button">
<input value="2" onclick="anadir('botón 2')" type="button">
<input value="3" onclick="anadir('botón 3')" type="button">
</body>
</html>