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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

HTML

            
              <!DOCTYPE html>
<html lang="es">
<head>
	<meta charset="UTF-8">
	<title>Unidades de medida diferencia entre font-size rem vs font-size em | CSS3</title>
	<link rel="stylesheet" href="css/medidas.css">
</head>
<body>
	<div class="wrapper">
		<div class="cont-bajada">
			<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Voluptatum dolor impedit a dolorem id! Tempora enim facere repellendus, consequuntur reiciendis aut, aperiam tenetur culpa corporis dicta, saepe tempore inventore facilis?</p>
		</div>
		<div class="cont-padre">
			<div class="cont-hijo">
				<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Placeat tempora, iste aperiam voluptatem doloremque laborum mollitia quis rerum quia, ut consectetur facere maxime quasi dolor quod, eos in veritatis harum!</p>
			</div>
		</div>
	</div>
</body>
</html>
            
          
!

CSS

            
              html{
	font-size: 20px;
}
.wrapper{
	display: table; 
	width: 100%; 
	max-width: 500px; 
	margin: auto;
	height: 600px; 
	background: #a4c739;
} 
.wrapper .cont-bajada{
	width: 100%; 
	height: 250px; 
	margin: auto; 
}
.wrapper .cont-bajada p{
	padding: 30px;
	font-size: 1rem;
}
.wrapper .cont-padre{
	width: 100%; 
	height: 250px; 
	margin: auto; 
}
.wrapper .cont-padre .cont-hijo{
	width: 12.5rem;
	height: 12.5rem;
	margin: auto;
	background: aqua;
}
.wrapper .cont-padre .cont-hijo p{
	padding: 30px;
	font-size: 1em;
}




/* === NOTAS === */
/*
1.- Unidades Relativas : px - rem - em - %

2.- Unidades Absolutas : in - cm - mm - pc - pt

3.- em : Relativo al tamaño de fuente que tenga especificada su elemento padre o el mas cercano;
(font-size : 20px; = font-size : 1em;)

4.- rem : Relativo al tamaño de fuente especificada en el body, html;

Para transformar de unidad de medida (px) a (rem-em) 
Ejemplo : 
30 / 20 = 1.5[rem-em];
[nueva medida] / [medida padre] = nueva medida rem-em;
*/
            
          
!

JS

            
              
            
          
!
999px

Console