Stacking order of elements affected by opacity - css

Question

How are z-index and opacity related when deciding stacking order of an element in HTML?
when i keep opacity less than 1 on an element which is having some z-index say 999. The element is going behind the element which is having no z-index.
$(function() {
$("#checkbox1").on("change", function() {
$("#green-parent").toggleClass("add-opacity", this.checked);
});
});
.green,
.blue {
position: absolute;
width: 100px;
line-height: 100px;
text-align: center;
color: white;
}
.green {
z-index: 999999999;
top: 50px;
left: 50px;
background: green;
}
.blue {
top: 60px;
left: 90px;
background: blue;
}
.add-opacity {
opacity: 0.99;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<input id="checkbox1" type="checkbox" value="1">
<label for="checkbox1">Add opacity to green box parent</label>
<div id="green-parent">
<span class="green">Green</span>
</div>
<div>
<span class="blue">Blue</span>
</div>

Answer 1

Positioned elements with a z-index value other than "auto" and elements with an opacity value less than 1 generate a stacking context. Refer to the rules regarding the painting order.
In your first example we have the root stacking context with various descendants including:
positioned green box with positive z-index
positioned blue box with auto z-index
The blue box with auto z-index is placed behind; green box with positive z-index is placed in front (see rule no. 8 and 9).
In your second example we have:
an element with opacity (which contains green box; note that z-index on the green box becomes local to this element)
positioned blue box without z-index
Both elements fall under same category (see rule no. 8). In which case the HTML order determines which element appear in front. The blue box appears later in the source order so it appears in front.

Answer 2

Aside from the opacity stacking context Alexey Ten pointed out in his comment (which is a factor here), the z-index is relative to the element's container. In this case, both your blue and green elements are contained within separate div parents which have no defined z-index. Due to the HTML ordering, the latter div (the one with the blue box) will appear on top of the former one (the green one).
In this below example, I've added the class .first to the first parent div and .second to the second one, then given them their own z-index properties.
.green, .blue {
position: absolute;
width: 100px;
color: white;
line-height: 100px;
text-align: center;
}
.green {
z-index:999999999;
top: 90px;
left: 60px;
background: green;
}
.gp{
opacity:0.99;
}
.blue {
top: 100px;
left: 100px;
background: blue;
}
.first, .second {
position: relative;
}
.first {
z-index: 2;
}
.second {
z-index: 1;
}
<div class="first">
<span class="green">Green</span>
</div>
<div class="second">
<span class="blue">Blue</span>
</div>