Why does clip-path (and other properties) affect the stacking order (z-index) of elements later in DOM?

Question

This has been asked before, but only where z-index is explictly defined in the CSS.

I am trying to use clip-path on a heading, but then pull up an imag

Answer 1

From the specifcation:

A computed value of other than none results in the creation of a stacking context the same way that CSS opacity does for values other than 1.

Then considering the painting order:

8. All positioned, opacity or transform descendants, in tree order that fall into the following categories:
   1. All positioned descendants with 'z-index: auto' or 'z-index: 0', in tree order. For those with 'z-index: auto', treat the element as if it created a new stacking context, but any positioned descendants and descendants which actually create a new stacking context should be considered part of the parent stacking context, not this new one. For those with 'z-index: 0' treat the stacking context generated atomically.
   2. All opacity descendants with opacity less than 1, in tree order, create a stacking context generated atomically.
   3. All transform descendants with transform other than none, in tree order, create a stacking context generated atomically.

The element with clip-path is painted at the step (8) and the image will be painted before if has no position set

4. For all its in-flow, non-positioned, block-level descendants in tree order: If the element is a block, list-item, or other block equivalent ...

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If you add position:relative to image it will be postioned and will fall under the step (8) and the tree order will decide making it above the clip-path element

Here is the same code with opacity where you will have the same painting order:

body {
  padding: 1em;
}

header {
  background: #a00;
  opacity:0.8;
}

h1 {
  margin: 0;
  padding: 2em;
  font: 300%;
  color: white;
  text-align: center;
}

section {
  background: #ccc;
  padding-top:5em;
  margin-top:-5em;
}

img {
  margin-top: -10em;
  
}

<header>
  <h1>Header Content</h1>
</header>

<section>
  <img src="https://via.placeholder.com/330/0000FF/808080"/>
</section>

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Related:

Why does position:relative; appear to change the z-index?

Why can't an element with a z-index value cover its child?

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Many other properties behave the same way and oblige your element to be painted at the step (8):

• filter ^ref
• backdrop-filter ^ref
• perspective ^ref
• mix-blend-mode ^ref