CSS children 之间的过渡

Question

快速提问。如果我将鼠标悬停在 child 上以瞄准另一个 child，或者一个单独的 div 与另一个单独的 div 是否有效？

只有将 children 放入容器中并将鼠标悬停在 parent 上，我才能过渡到工作状态。这是什么规则？

示例 1（不起作用）：

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  display: flex;
  justify-content: center;
}

.underline {
  width: 0px;
  height: 2px;
  background-color: black;
  transition: 0.4s;
}

h2:hover .underline {
  width: 165px;
}

<body>

    <h2>Hover Over Me</h2>
    <div class="underline"></div>

</body>

示例 2（不起作用）

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  display: flex;
  justify-content: center;
}

.container {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  width: 180px;
  height: 50px;
}

.underline {
  width: 0px;
  height: 2px;
  background-color: black;
  transition: 0.4s;
}

h2:hover .underline {
  width: 165px;
}

<body>

  <div class="container">
    <h2>Hover Over Me</h2>
    <div class="underline"></div>
  </div>

</body>

示例 3（有效）

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  display: flex;
  justify-content: center;
}

.container {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  width: 180px;
  height: 50px;
}

.underline {
  width: 0px;
  height: 2px;
  background-color: black;
  transition: 0.4s;
}

.container:hover .underline {
  width: 165px;
}

<body>

  <div class="container">
    <h2>Hover Over Me</h2>
    <div class="underline"></div>
  </div>

</body>

Answer 1

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  display: flex;
  justify-content: center;
}

.container {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  width: 180px;
  height: 50px;
}

.underline {
  width: 0px;
  height: 2px;
  background-color: black;
  transition:0.4s;
}

h2:hover + .underline {
  width: 165px;
}

<body>

  <div class="container">
    <h2>Hover Over Me</h2>
    <div class="underline"></div>
  </div>

</body>

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