<div id="error" class="warning" style="background: pink;">
  내가 무슨 색일까요?
</div>
div {
  background: blue; /* 명시도: 0,0,0,1 */
}

#error {
  background: red !important; /* ❗❗ */
}

.warning {
  background: yellow !important; /* ❗ */
}

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