* {
margin: 0px;
padding: 0px;
}

h1 {
font: bold 20px verdana, sans-serif;
}

h2 {
font: bold 14px verdana, sans-serif;
}

h3 {
font: bold 12px verdana, sans-serif;
}


header, section, footer, aside, nav, article, figure, figcaption,
hgroup, div{
display: block;
}

body {
text-align: center;
}

#agrupar {
width: 960px;
/*width: 110%;*/
margin: 15px auto;
text-align: left;
border: 1px solid #000000;
/*display: block;*/
}

#cabecera {
background: #ffffff;
border: 1px solid #999999;
padding: 20px;
}

#menu {
background: #ededed;
padding: 5px 5px;
border-top: 2px solid #999999;
border-bottom: 2px solid #999999;
color:gray;
text-decoration: none;
}

#menu li {
display: inline-block;
list-style: none;
padding: 5px;
font: bold 12px verdana, sans-serif;
color:gray;
text-decoration: none;
border-radius: 2px solid #999999;
}

<!--Para el formato de la tabla del menú:-->

body {margin: 0;}

ul.topnav {
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
  background-color: #566573; /*Color gris oscuro*/
}

ul.topnav li {float: left;}

ul.topnav li a {
  display: block;
  color: white;
  text-align: center;
  /*padding: 14px 16px;*/
  padding: 5px 2px;
  text-decoration: none;
}
/*color de fondo al mousehover*/
ul.topnav li a:hover:not(.active) {background-color: #F57C00;} /*Color naranja*/

ul.topnav li a.active {background-color: #4CAF50;} /*Color verde*/

ul.topnav li.right {float: right;}

@media screen and (max-width: 600px) {
  ul.topnav li.right, 
  ul.topnav li {float: none;}
}


/*<!--Menú desplegable-->*/

li a, .dropbtn {
  display: inline-block;
  color: white;
  text-align: center;
  padding: 7px 8px;
  text-decoration: none;
}

li a:hover, .dropdown:hover {
  background-color: orange; /*contenido de la 1era opción*/
}

li.dropdown {
  display: inline-block;

}

.dropdown-content {
  display: none;
  position: absolute;
  background-color: green;
  min-width: 120px;
  max-width: 170px;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
  z-index: 1;
  font-size: 12px;
}

.dropdown-content a {
  color: black; /*LETRAS DEL CONTENIDO*/
  padding: 12px 16px;
  text-decoration: none;
  display: block;
  text-align: left;
}

.dropdown-content a:hover {background-color: #f1f1f1;}

.dropdown:hover .dropdown-content {
  display: block;
  background-color: white;

}



#seccion {
/*float: right;
max-width: 60%;
margin: 1%;
font: 12px verdana, sans-serif;*/

float: right;
width: 660px;
margin: 20px;
font: 12px verdana, sans-serif;
}

#columna {
float: left;
/*width: 220px;
height: 650px;*/
width: 33%;
height: 50%;
margin: 2%;
padding: 20px;
background-color: #ededed;
font: 11px verdana, sans-serif;
}

#pie {
clear: both;
text-align: center;
padding: 10px;
border-top: 2px solid #999999;
background-color: #ededed;
font-color: black;
font-size: 10px;
font: verdana, sans-serif;
}

article {
background: #ffffff;
border: 1px solid #999999;
padding: 20px;
margin-bottom: 15px;
}

article footer {
text-align: right;
}

time {
color: #999999;
}

figcaption {
font: italic 14px verdana, sans-serif;
}

img{
	box-shadow: 5px 5px gray;
}
/*
-moz- para Firefox.
-webkit- para Safari y Chrome.
-o- para Opera.
-khtml- para Konqueror.
-ms- para Internet Explorer.
*/

/*div {
width: 100px;
margin: 20px;
padding: 10px;
border: 1px solid #000000;

-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
box-sizing: border-box;

}*/

#btnRegistro_Usuario{
width: 130px;
float: right;
background: #ededed;
padding: 2px 5px;
border-radius: 2px solid #999999;
color:gray;
font: bold 10px verdana, sans-serif;
text-decoration: none;
margin-right: 15px;	
}

#btnIniciar_Sesion{
width: 130px;
float: right;
background: #4CAF50;
/*background: #ededed;*/
padding: 2px 5px;
border-radius: 2px solid #999999;
color:white;
font: bold 10px verdana, sans-serif;
text-decoration: none;	
}

#redes_sociales {
border-collapse:collapse; margin: 0 auto;}
}
#redes_sociales th {
border-collapse:collapse; 
margin: 0 auto;
padding:5px;
text-align: justify-all;	
}

/*Redes sociales con css (ver index en respaldo cuando regrese el internet*/
/*#icon-bar{
  position: fixed;
  top: 50%;
  -webkit-transform: translateY(-50%);
  -ms-transform: translatey(-50%);
}
#icon-bar a{
  display: block;
  text-align: center;
  padding:16px;
  transition: all 0.3s ease;
  color: white;
  font-size:20px;
}*/