h1 { color: red; text-align: center; }
h2 { color: blue; }
nav { text-align: right; }
nav li {
  display: inline;
  border: 1px solid blue;
  padding: 5px;
  background-color: lime;
}
a:hover { color: red; background-color: aqua; }
a:focus { outline: 2px solid blue; }
h2 + p { color: red; }
img { display: none; }
.xem:active + img { display: block; }
em.ten { border: 1px solid teal; font-style: italic; }