How is the rendering order of elements determined when they overlap each other (z-index, stacking context)?

The order of rendering elements when they overlap is determined by the stacking context mechanism and the z-index property.

Stacking context is an isolated group of elements within which the ordering along the Z-axis (depth) occurs. Each stacking context has its own local stacking order.

Key points:

• A new stacking context is created, for example, when there is positioning (position: relative/absolute/fixed) and z-index, opacity less than 1, transform, filter, and other CSS properties.
• Elements within the same stacking context are ordered by z-index (a numerical value). The higher the z-index, the higher the element is in the stack.
• If elements belong to different stacking contexts, the order is determined by the nesting and creation order of these contexts.

Example:

.parent {
  position: relative;
  z-index: 1; /* creates a stacking context */
}
.child1 {
  position: absolute;
  z-index: 10;
}
.child2 {
  position: absolute;
  z-index: 5;
}

Here, .child1 will be above .child2 within .parent. But if there is another stacking context with a higher level, it can overlay these elements regardless of their z-index.

Thus, understanding where stacking contexts are created and how z-index affects elements within them is crucial for managing rendering order.