How Does Opacity Affect Z-Index in Browser Rendering?

Z-Index vs. Opacity in Browser Rendering

In the realm of web development, controlling the positioning and visibility of elements is crucial for creating visually appealing and functional user interfaces. Two key properties in CSS that influence these aspects are opacity and z-index. However, understanding their interplay can be a bit tricky.

The question at hand arises when a "popup window" obscures a partially transparent navy square on a web page. Despite setting a higher z-index for the popup, the square remains visible underneath, even though it has an opacity of less than 1.

This seemingly paradoxical behavior stems from a fundamental principle in browser rendering:

Opacity and Stacking Contexts

When an element has an opacity less than 1, it is considered transparent. According to the visual formatting module of CSS, a new stacking context is created for such elements. This context isolates the transparent element and its contents from the ordering of other elements in the document.

The Catch

Here's the catch: if the transparent element is not positioned, it's painted in its parent stacking context as if it had a z-index of 0 and opacity of 1. This means that it appears below any positioned elements, regardless of their z-index. Only when the transparent element is positioned itself does the z-index property take effect.

Implications

Understanding this behavior is crucial when layering elements with varying opacity and positioning. For instance, if you have a background image with transparency and want transparent elements to appear in front of it, you need to ensure that those elements are positioned. Otherwise, they will be obscured by the opaque parts of the background.

Clarification

Contrary to what the question might imply, opacity does have a higher priority than z-index for non-positioned elements with opacity less than 1. It's essential to take this into account when designing your web pages to achieve the desired visual effects.

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