Hola:
EJERCICIO
Crea un documento HTML con 4 elementos div de 400 píxeles de ancho y 400 píxeles de alto, todos ellos con un margin de 40 píxeles en todas direcciones y un padding de 40 píxeles en todas direcciones. En cada uno de los elementos div crea un borde y coloca una imagen de fondo diferente y un background-color diferente. Usa la propiedad background-position para hacer que la imagen esté centrada tanto vertical como horizontalmente respecto al borde del div (por ejemplo, si una imagen mide 100x100 píxeles, deberá existir la misma distancia hasta el borde del div en las cuatro direcciones).
Yo lo resolví de la siguienta manera:
HTML
<!DOCTYPE html/>
<html lang="es">
<html>
<title>Ejercicio background</title>
<meta charset="UTF-8"/>
<link rel="stylesheet" type="text/CSS" href="ejerciciobackground.css"/>
<head>
</head>
<body>
<div id="primero"></div>
<div id="segundo"></div>
<div id="tercero"></div>
<div id="cuarto"></div>
</body>
</html>
CSS
div{
width:400px;
height:400px;
margin:40px;
padding:40px;
background-repeat:no-repeat;
background-position:50% 50%;
}
#primero{
background-image:url(data:image/jpeg;base64,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);
background-color:orange;
border:solid 5px blue;
}
#segundo{background-image:url(data:image/jpeg;base64,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);
background-color:cyan;
border:dotted 3px #FF0000;
}
#tercero{background-image:url(data:image/jpeg;base64,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);
background-color:lightgreen;
border:dashed 7px #00DD00;
}
#cuarto{background-image:url(data:image/jpeg;base64,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);
background-color:yellow;
border:groove 10px #0000EE;
}
Una cosa que me resultó extraña, es que inicialmente yo había identificado los elementos div utilizando números simples (1,2,3 y 4) y no lograba visualizarlos en el navegador. Sólo cuando les cambié los id a "primero, segundo, etc.", pude visualizarlos. No sé cuál puede ser la causa de esto.