Why Do Semi-Transparent Elements Seem to Override Higher Z-Index Elements?

Transparent Elements and Z-Index in Browsers

Question

When using CSS, an element with an opacity value less than 1 appears to override a positioned element with a higher z-index, causing the positioned element to remain partially visible. Why does this occur?

Answer

This behavior is a result of the way that browsers handle transparency. When an element has an opacity value less than 1, it is composited offscreen, creating a new stacking context. If the element is not positioned, it is rendered in the parent stacking context as if it had a z-index of 0 and opacity of 1.

Therefore, even if a positioned element has a higher z-index, it cannot appear on top of an unpositioned, semi-transparent element. To prevent this behavior, it is recommended to position the semi-transparent element and adjust its z-index value accordingly.

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