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<!DOCTYPE html>
<html lang="es">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Prueba</title>
</head>
<body>
<form action="#" method="post">
<input type="search" name="buscar" id="buscar" value="Buscar..." size="20">
</form>
<script src="funciones.js"></script>
</body>
</html>
document.getElementById('buscar').addEventListener('focus', (e) => {
e.target.value = ''
})