onmouseout

css/style.css

* {
margin: 0;
padding: 0;
}
.a {
height: 400px;
width: 400px;
position: absolute;
top: 50%;
left: 50%;
margin-top: -200px;
margin-left: -200px;
background-color: #7092BE;
color: #FFF;
text-align: center;
line-height: 400px;
font-size: 50px;
}

* {
margin: 0;
padding: 0;
}
.a {
height: 400px;
width: 400px;
position: absolute;
top: 50%;
left: 50%;
margin-top: -200px;
margin-left: -200px;
background-color: #7092BE;
color: #FFF;
text-align: center;
line-height: 400px;
font-size: 50px;
}

js/script.js

function funMouseOver() {
var val = document.getElementById('d1');
val.className = 'a';
val.style.backgroundColor = '#3BA9DE';
val.innerHTML = 'Привет Мир!';
}
function funMouseOut() {
var val = document.getElementById('d1');
val.className = 'a';
val.style.backgroundColor = '#7092BE';
val.innerHTML = 'Hello World!';
}
window.onload = function() {
document.getElementById('d1').onmouseover = funMouseOver;
document.getElementById('d1').onmouseout = funMouseOut;
}

function funMouseOver() {
var val = document.getElementById('d1');
val.className = 'a';
val.style.backgroundColor = '#3BA9DE';
val.innerHTML = 'Привет Мир!';
}
function funMouseOut() {
var val = document.getElementById('d1');
val.className = 'a';
val.style.backgroundColor = '#7092BE';
val.innerHTML = 'Hello World!';
}
window.onload = function() {
document.getElementById('d1').onmouseover = funMouseOver;
document.getElementById('d1').onmouseout = funMouseOut;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script src="js/script.js"></script>
</head>
<body>
<div id="d1" class="a">Hello World!</div>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script src="js/script.js"></script>
</head>
<body>
<div id="d1" class="a">Hello World!</div>
</body>
</html>