<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
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            width: 100%;
            height: 500px;
            background-color: black;
        }
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</head>
<body>
    <div id="fk"></div>
</body>
<script>
    var a=document.getElementById('fk');
    
    a.onmouseover = function(){
        a.style.background = 'red';
        a.style.width='50%';
        a.style.height='300px';
        
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        a.style.width='100%';
        a.style.height='500px';
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</script>
</html>