如何在复杂的 HTML 结构中选择具有特定类的第一个元素？

How to Select First Element of a Given Class

When working with HTML elements, selecting specific elements for styling or manipulation is crucial. One scenario involves targeting the first occurrence of a class within another element, presenting a challenge when the position of the desired class varies within different elements.

In this context, we aim to select the first element with the class 'A' within an element identified by id or class 'B'. The problem arises when 'B' may differ in both name and structure, and the 'A' element is inconsistent in its position within 'B'. However, one consistent factor remains: the presence of an outer element 'C'.

Solution: Utilizing Sibling Selectors

CSS3 introduces the :first-of-type pseudo-class, which selects the first element of its type among siblings. Unfortunately, CSS lacks a :first-of-class pseudo-class. Therefore, we need an alternative workaround.

One approach involves utilizing the general sibling combinator (~) along with overriding styles to achieve our goal. This technique leverages the fact that CSS applies styles in a top-down manner. By defining two rules, we can override the default styling of '.A' elements that do not match our desired criteria:

.C > * > .A {
  /* Styles for all '.A' elements that are grandchildren of '.C' */
}

.C > * > .A ~ .A {
  /* Styles for '.A' elements that follow the first '.A' child of any element that is a child of '.C' */
}

In these rules, the first targets all '.A' grandchildren of '.C'. Since this includes the first occurrence we want, we need to "undo" this styling for subsequent '.A' elements using the second rule. This is accomplished by using the ~ combinator to select only those '.A' elements that follow another '.A' element under the same parent.

How Styles Are Applied

To illustrate how this technique works, consider the following HTML structure:

<div class="C">
  <div class="B">
    <div class="E">Content <!-- [1] --></div>
    <div class="F">Content <!-- [1] --></div>
    <div class="A">Content <!-- [2] --></div>
    <div class="A">Content <!-- [3] --></div>
  </div>
  <!-- ... other elements ... -->
</div>

Here, '[1]' represents elements that do not meet our criteria, while '[2]' and '[3]' represent the first and any subsequent '.A' elements we want to style differently.

1. The first rule styles all '.A' elements that are grandchildren of '.C'. This includes elements '[2]' and '[3]'.
2. The second rule targets any '.A' element that follows another '.A' element under the same parent. This includes element '[3]' but not '[2]' because it has no preceding sibling with class '.A'.

As a result, element '[2]' (the first '.A') retains the default styling, while element '[3]' has its styling overridden by the second rule. Thus, we successfully target and style the first occurrence of class '.A' within the context of element '.C'.

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