html {
  font-size: 1em;
  line-height: 1.5em;
  font-family: "Verdana", Helvetica, sans-serif;
}

body {
  margin: 0;
  background: #f3f3f3;
}

.encabezado {
  text-align: center;
  background: #006699;
  border-bottom: 0.1em solid silver;  
  padding: 0.5em;
  }


.footer-principal {
  text-align: center;
  background: #006699;
  border-bottom: 0.1em solid silver;  
  padding: 1em;
  }

.principal {
  max-width: 900px;
  margin: auto;
  padding: 1em;
}

.caja1, .caja3 {
  background: #fff;
  border: 0.1em solid gray;
  padding: 1em;
  margin: 1em;
  box-shadow: 0 0 0.1em 0.01em black;
} 


.caja2 {
  background: #fff;
  border: 0.08em solid gray;
  padding: 1em;
  margin: 1em;
  box-shadow: 0 0 0.1em 0.01em black;
  border-radius: 15px;
} 

.caja1 h2 {
  font-family: 'Verdana', sans-serif;
  color: darkblue;
  text-shadow: 0.03em 0.03em 0.01em #000;
  letter-spacing: 0.03em;
}
.caja1 p, ol {
  font-family: 'Verdana', sans-serif;
  color: darkblue;
}

.caja2 h3 {
font-family: 'Roboto', sans-serif;
font-size: 1.2em;
text-align:center;
color: darkblue;
}

.caja2, .caja3 h2 {
  font-family: 'Verdana', sans-serif;
  color: darkblue;
  text-shadow: 0.03em 0.03em 0.01em #000;
  letter-spacing: 0.03em;
}

.articulo {
  margin-top: 2em;
}

main {
  display: block;
  margin: auto;
  padding: 0.50em;
  width: 100%;
}

span {
  display: block;
}

sup,
sub {
  font-size: 0.5em;
}

.calculadora {
  background: #c6eaff;
  margin: auto;
  padding: 0.50em;
  max-width: 400px;
  border: 1px solid;
  border-radius: 5px;
}

.pantalla,
.botones {
  display: flex;
}

input {
  margin: 0.50em auto;
  padding: 0 0.25em;
  text-align: right;
  font-size: 2.5em;
  letter-spacing: 0.05em;
  font-family: 'VT323', monospace;
  border-radius: 5px;
  width: 90%;
}

input[type=number]::-webkit-inner-spin-button, 
input[type=number]::-webkit-outer-spin-button { 
  -webkit-appearance: none; 
  margin: 0; 
}

button {
  flex: 1;
  padding: 0.50em 0;
  margin: 0.25em;
  text-align: center;
  font-size: 1.2em;
}

#alcuadrado,
#inverso,
#sqrt,
#elevado {  
  border: 1px solid silver;
  border-radius: 3px;
  background-color: #cccccc;
}

#alcuadrado:hover,
#inverso:hover,
#sqrt:hover,
#elevado:hover{
  border: 1px solid silver;
  background-color: #656565;
}

#vacio {
  border: 1px solid #c6eaff;
  border-radius: 3px;
  background-color: #c6eaff; 
}

#uno, #dos, 
#tres, #cuatro,
#cinco, #seis,
#siete, #ocho,
#nueve, #cero,
#borrar, #igual,
#decimal, #entero,
#pos-neg,
#suma, #resta,
#multiplica, #divide
 {
  border: 1px solid silver;
  border-radius: 3px;
  background-color: #f2f5f6;
}

#uno:hover, #dos:hover,
#tres:hover, #cuatro:hover,
#cinco:hover, #seis:hover,
#siete:hover, #ocho:hover,
#nueve:hover, #cero:hover,
#borrar:hover, #igual:hover,
#decimal:hover, #entero:hover,
#pos-neg:hover,
#suma:hover, #resta:hover,
#multiplica:hover, #divide:hover {
  border: 1px solid silver;
  background-color: #d4dee1;
}