Hola Pedro!
Todo perfecto
Para ver las tablas más separadas le he añadido table{margin-top:50px;}
Otra opción que también es viable es no meter las tablas dentro de div, sino aplicarles estilos directamente a las tablas:
<html>
<head> <title>Portal web - aprenderaprogramar.com</title> <meta charset="utf-8">
<style type="text/css">
/* Curso CSS estilos aprenderaprogramar.com*/
body {font-family: Arial, Helvetica, sans-serif; }
table{margin-top:50px;}
#tabla1 {width: 600px;}
#tabla1, #tabla1 th, #tabla1 td{border: solid 8px gray; border-collapse: collapse;}
#tabla1 td, #tabla2 th{width: 20%;}
#tabla2 {width: 100%;}
#tabla2, #tabla2 th, #tabla2 td{border: solid 2px brown; border-collapse: separate;}
#tabla2 td, #tabla2 th{width: 20%;}
#tabla3 {width: 500px;}
#tabla3, #tabla3 th, #tabla3 td{border-bottom: solid 6px blue; border-collapse: collapse;}
#tabla3 td, #tabla3 th{width: 100px;}
</style>
</head>
<body>
<table id="tabla1">
<caption>Tabla 1</caption>
<tr>
<th>Encabezado 1</th><th>Encabezado 2</th><th>Encabezado 3</th><th>Encabezado 4</th><th>Encabezado 5</th>
</tr>
<tr>
<td>fila 1</td><td>fila 1</td><td>fila 1</td><td>fila 1</td><td>fila 1</td>
</tr>
<tr>
<td>fila 2</td><td>fila 2</td><td>fila 2</td><td>fila 2</td><td>fila 2</td>
</tr>
<tr>
<td>fila 3</td><td>fila 3</td><td>fila 3</td><td>fila 3</td><td>fila 3</td>
</tr>
<tr>
<td>fila 4</td><td>fila 4</td><td>fila 4</td><td>fila 4</td><td>fila 4</td>
</tr>
<tr>
<td>fila 5</td><td>fila 5</td><td>fila 5</td><td>fila 5</td><td>fila 5</td>
</tr>
<tr>
<td>fila 6</td><td>fila 6</td><td>fila 6</td><td>fila 6</td><td>fila 6</td>
</tr>
</table>
<hr/>
<table id="tabla2" >
<caption>Tabla 2</caption>
<tr>
<th>Encabezado 1</th><th>Encabezado 2</th><th>Encabezado 3</th><th>Encabezado 4</th><th>Encabezado 5</th>
</tr>
<tr>
<td>fila 1</td><td>fila 1</td><td>fila 1</td><td>fila 1</td><td>fila 1</td>
</tr>
<tr>
<td>fila 2</td><td>fila 2</td><td>fila 2</td><td>fila 2</td><td>fila 2</td>
</tr>
<tr>
<td>fila 3</td><td>fila 3</td><td>fila 3</td><td>fila 3</td><td>fila 3</td>
</tr>
<tr>
<td>fila 4</td><td>fila 4</td><td>fila 4</td><td>fila 4</td><td>fila 4</td>
</tr>
<tr>
<td>fila 5</td><td>fila 5</td><td>fila 5</td><td>fila 5</td><td>fila 5</td>
</tr>
<tr>
<td>fila 6</td><td>fila 6</td><td>fila 6</td><td>fila 6</td><td>fila 6</td>
</tr>
</table>
<hr/>
<table id="tabla3">
<caption>Tabla 3</caption>
<tr>
<th>Encabezado 1</th><th>Encabezado 2</th><th>Encabezado 3</th><th>Encabezado 4</th><th>Encabezado 5</th>
</tr>
<tr>
<td>fila 1</td><td>fila 1</td><td>fila 1</td><td>fila 1</td><td>fila 1</td>
</tr>
<tr>
<td>fila 2</td><td>fila 2</td><td>fila 2</td><td>fila 2</td><td>fila 2</td>
</tr>
<tr>
<td>fila 3</td><td>fila 3</td><td>fila 3</td><td>fila 3</td><td>fila 3</td>
</tr>
<tr>
<td>fila 4</td><td>fila 4</td><td>fila 4</td><td>fila 4</td><td>fila 4</td>
</tr>
<tr>
<td>fila 5</td><td>fila 5</td><td>fila 5</td><td>fila 5</td><td>fila 5</td>
</tr>
<tr>
<td>fila 6</td><td>fila 6</td><td>fila 6</td><td>fila 6</td><td>fila 6</td>
</tr>
</table>
</body>
</html>
Saludos!