محرر ويب أونلاين

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<!DOCTYPE html>
<html>
    <head>
        <style>
* { 
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}
            
body {
    display: flex;
    min-height: 100vh;
    align-items: center;
}
            
div {
    margin: auto;
    border: 1px solid gray;
}
        </style>
    </head>
    <body>
        <div>Element is centered horizontaly.</div>
    </body>
</html>