:focus

:focus {
  /* ... */
}

<div><input class="red-input" value="当获得焦点时我将呈现红色。" /></div>
<div><input class="blue-input" value="当获得焦点时我将呈现蓝色。" /></div>

.red-input:focus {
  background: yellow;
  color: red;
}

.blue-input:focus {
  background: yellow;
  color: blue;
}