Pseudo-element: It looks like a child element, not exactly like this

Let's look at a container containing child elements:

<div class="container">
  <div>item</div>
  <div>item</div>
  <div>item</div>
</div>

If we add the following style:

 .container::before {
  content: "x";
}

The effect is equivalent to:

<div class="container">
  [[[ ::before pseudo-element]]]
  <div>item</div>
  <div>item</div>
  <div>item</div>
</div>

The behavior of pseudo-elements is similar to that of child elements in most cases. However, an important difference is that no other selector can directly select it except for the selector that creates it (or similar selectors such as ::before or ::after ).

For example, suppose we set the container to a 2x3 grid, with each child element being a pill-like design:

 .container {
  display: grid;
  grid-template-columns: 1fr 1fr;
  grid-gap: 0.5rem;
}

.container > * {
  background: darkgray;
  border-radius: 4px;
  padding: 0.5rem;
}

If there are no pseudo-elements, the effect is as expected. But after adding the pseudo-element, you will find that it participates in the grid layout, which may be unexpected. Pseudo-elements are often used for decorative purposes, and it seems a bit strange to participate in the content layout.

Note that .container > * selector does not make the pseudo-element dark gray as well, because you cannot select the pseudo-element this way. This is another small issue to note.

In daily development, pseudo-elements usually use position: absolute for decorative operations:

 .container::before {
  content: "";
  position: absolute;
  /* Decorative style*/
}

In this case, you may not notice the existence of the pseudo-element at all. Although it is still technically a child element, participating in the DOM structure, it does not participate in the grid layout. This is not a phenomenon unique to CSS Grid. In Flexbox, pseudo-elements will also become Flex projects. You can control pseudo-elements as you like using floating or other layout methods.

Developer tools clearly show that pseudo-elements are similar to child elements in the DOM.

There are some other issues to note:

One is :nth-child() selector. You might think that if pseudo-elements are child elements, they will affect the :nth-child() calculation, but that is not the case. This means:

 .container > :nth-child(2) {
  background: red;
}

The same element is selected regardless of whether the ::before pseudo-element exists or not. The same is true for selectors such as ::after , :nth-last-child , etc. This is why "kinda" in the title. If pseudo-elements are exactly like child elements, they affect these selectors.

Another problem is that you cannot select pseudo-elements in JavaScript like you would choose normal child elements. document.querySelector(".container::before"); will return null . If you need to access the style of pseudo-elements in JavaScript, you can use CSSOM:

 const styles = window.getComputedStyle(
  document.querySelector('.container'),
  '::before'
);
console.log(styles.content); // "x"
console.log(styles.color); // rgb(255, 0, 0)
console.log(styles.getPropertyValue('color')); // rgb(255, 0, 0)

Have you encountered other issues with pseudo-elements?

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