Why Doesn't the CSS :after Pseudo-element Work on All HTML Elements?

Unveiling the Constraints of CSS :after Property

The CSS :after pseudo-element adds content after an element's content in the document tree. However, its applicability raises questions.

Element Restrictions

The CSS spec states that :after can be applied to content before or after an element's tree content, seemingly without restrictions. Yet, in practice, certain elements, such as images (), inputs (), and tables (

Understanding Replaced Elements

The key to understanding this behavior lies in the concept of replaced elements. Replaced elements are those whose appearance and dimensions are defined by an external resource, including images, plugins, and form elements.

:after and Replaced Elements

Crucially, the CSS spec explicitly states that :before and :after only work with non-replaced elements. Therefore, replaced elements like and cannot have :after or :before properties.

DOM Structure Considerations

When using :before and :after with non-replaced elements, the DOM structure looks like this:

Content of span

For replaced elements like images, this structure is not feasible, hence the inconsistency in behavior.

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