How Does `position: relative` Affect Element Overlap and Z-index?

Why Does position: relative; Seem to Alter Z-Index?

When you're working with a web layout, setting an element to position: relative may appear to change its z-index and affect how it overlaps with other elements. However, it's important to understand the underlying mechanics of the paint order to clarify what's actually happening.

Without position: relative, the element isn't positioned and is rendered during the painting step reserved for non-positioned elements. However, elements with position: relative are considered "positioned" and are painted in a subsequent step.

Now, when you place the position: relative rule on the container, it becomes a positioned element as well. According to the painting order, positioned elements are rendered in the order they appear in the HTML code. Since the container is rendered after the .mask element, it will appear above the blue overlay.

If you were to switch the order of the elements in the HTML code, putting the .mask element after the container, the effect of position: relative on z-index would disappear. That's because the two elements would still be painted in the same order due to the tree order.

In summary, position: relative doesn't directly alter z-index but instead influences the painting order of elements. By understanding the painting steps, we can accurately predict how elements will overlap and adjust our layout accordingly.

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