How Does `position:relative;` Seem to Affect Z-index?

Understanding the Effects of position:relative; on Z-Index

In certain scenarios, using "position:relative;" on an element appears to modify its z-index, leading to confusion. This answer aims to clarify this behavior by referring to the CSS painting order specification.

CSS Painting Order

The CSS painting order dictates the sequence in which HTML elements are painted onto the screen. For non-positioned elements (i.e., without "position:absolute;" or "position:relative;" set), they are painted in order of the HTML markup (step 4). However, positioned elements (such as .mask in the provided code example) are painted later, after all non-positioned elements (step 8).

Relative Positioning Impact

When position:relative; is applied to an element (such as .container), it becomes positioned relative to its normal flow. Consequently, it is removed from the step 4 painting order, falling instead under step 8 with other positioned elements.

In the given code, .mask is painted in step 8 (as it is absolutely positioned), while .container (without position:relative;) would be painted in step 4. Thus, .mask appears in front of .container, as expected.

However, if position:relative; is applied to .container, it also falls under the step 8 painting order. Since both .mask and .container are positioned and have no specified z-index, the element that appears later in the document (i.e., .container) will be painted on top of the other element (i.e., .mask).

Conclusion

By understanding the CSS painting order, it becomes clear why setting position:relative; on a container element can appear to affect its z-index. This behavior is due to the element becoming positioned and thus being painted later in the rendering process, resulting in a change in visual ordering relative to other positioned elements.

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