How Does `position:relative` Impact Z-Index and Element Stacking Order in CSS?

Why the "position:relative" Attribute Alters the Z-Index Behavior

In HTML and CSS, the "position:relative" property is frequently used to position elements within their container. However, in certain scenarios, it can seem as though this property also affects the z-index, even though it is not explicitly stated.

To understand this behavior, it is necessary to delve into the CSS painting order. According to the CSS specification, elements are painted in the following sequence:

1. Normal flow: Elements are painted in the order they appear in the HTML document.
2. Floats: Floating elements are painted in tree order after the normal flow.
3. Positioned elements: Elements with position values other than "static" are painted in tree order after floats.
4. In-flow non-positioned block elements: These elements are painted in tree order after positioned elements.
5. Absolutely positioned elements: Elements with position "absolute" are painted in tree order after normal flow, floats, and positioned elements.

By default, an element without any explicit position specified (such as "position:static" or "position:absolute") is considered "in-flow" and will be painted during step 4. However, if that element's parent container is given "position:relative," it will become a positioned element and will be painted during step 3.

In the given example, if the ".container" element does not have "position:relative," then the ".mask" element, which has "position:absolute," will be painted on top of it during step 5 (after positioned elements). However, when "position:relative" is applied to the ".container," it becomes a positioned element and is therefore painted during step 3. As such, the ".container," along with its children, will be painted before the ".mask" element, resulting in the blue overlay appearing beneath the text.

It's important to note that the order in which elements are painted in the DOM (Document Object Model) does not necessarily correspond to the order in which they appear visually. The specified z-index values take precedence in determining which element appears on top. However, if no z-index is specified, the painting order as described above will be used.

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