How Does Opacity Affect Z-Index and Stacking Order in Browser Rendering?

Z-Index vs. Opacity: Unraveling the Stacking Order in Browsers

When creating layered content in JavaScript, it's important to understand how different CSS properties affect the stacking order. One surprising behavior is the interaction between opacity and z-index.

In a hypothetical scenario, a "popup window" is superimposed over a navy square with an opacity of 0.3. Intuitively, one would expect the popup to obscure the square completely, but part of the square remains visible.

This behavior can be attributed to a fundamental aspect of browser rendering. When an element has an opacity value below 1, it exists in a separate stacking context. This means that content outside the element cannot be layered between content within the element, regardless of z-index values.

In the case of the navy square, its opacity of 0.3 creates a new stacking context. As a result, the popup element, despite its higher z-index, is rendered below the square. When the opacity of the square is increased to 1, it exits the separate stacking context and the z-index takes effect, obscuring the square completely.

This nuance in the CSS rendering engine highlights the importance of considering stacking contexts when designing layered content in web pages.

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