In HTML structure, how to select the first child element with a specific class name through CSS?

In HTML structure, precise positioning of the first child element with a specific class name is a common CSS problem, especially when the number of child elements is uncertain. This article will demonstrate through examples how to solve this problem using the advanced features of the CSS selector.

Suppose we have a div container containing multiple activebar and item class elements, and the number of activebar and item elements is dynamically changed. Our goal is to select the first item class element only through CSS.

HTML structure example:

<div class="main">
  <div class="activebar"></div>
  <div class="activebar"></div>
  <div class="activebar"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

The simple first-child or first-of-type pseudo-class selector does not apply here, because first-child only selects the first child element of the parent element, while first-of-type only selects the first of the same type elements.

Workaround: Use the extended functionality of nth-child selector. Modern CSS allows the use of complex selectors in nth-child to define the scope.

The following CSS code allows you to accurately select the first item element:

 .main > .item:nth-child(n of .item) {
  /* Style rules*/
}

The function of nth-child(n of .item) is: among all the child elements of .main element, only those child elements that have the .item class are considered, and the first one is selected from it ( n represents the first one). This effectively ignores the activebar element in it and directly locates the first item element.

In this way, even if the number of activebar and item elements changes, we can always accurately select the first item element, thereby achieving flexible style control.

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