golden_yuna,
на всякий случай
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
#focus{
border: 1px dashed Gray; padding: 5px; height: 100px; width: 100px ; background-color: #A9A9A9;
}
</style>
<script>
window.addEventListener("DOMContentLoaded", function() {
var node = document.getElementById("checkFocus"),
dat = document.getElementById("focus");
node.addEventListener("change", function() {
dat.style.display = this.checked ? "block" : "none"
})
});
</script>
</head>
<body>
<input id="checkFocus" type="checkbox" checked="true"/>
<div id="focus"></div>
</body>
</html>