Element with negative margin covered by previous one - css

Question

I am trying to make element position with negative margin - one element should cover a bit the previous one. But it's bit complicated - negative margin should have only child div of #content. See example below. I need to have red element, than yellow element and gray element inside the red one should have negative margin and cover bottom of red one.
I have already tried relative and absolute position, change z-index, but none of these worked for me.
#sectionTitle {
height: 150px;
text-align: center;
margin: 0 auto;
width: 100%;
position: relative;
z-index: 1;
background: red;
}
#content {
background: yellow;
position: relative;
width: 100%;
height: auto;
min-height: 300px;
overflow: auto;
z-index: 20;
}
#content .block {
position: relative;
background: #eee;
width: 90%;
top: -50px;
margin: 0 auto 0 auto;
box-sizing: border-box;
height: auto;
min-height: 400px;
z-index: 30;
}
<div id="sectionTitle">
...some text...
</div>
<div id="content">
<div class="block">
...element which should cover bottom of #sectionTitle element
</div>
</div>
Now when I inspect the element, .block goes 50px to #sectionTitle, but it's not visible, because sectionTitle element covers it.

Answer 1

Simply remove z-index (to avoid the creation of a stacking context1) and overlow:auto (to avoid the creation of a block formatting context2) from the #content div:
#sectionTitle {
height: 150px;
text-align: center;
margin: 0 auto;
width: 100%;
position: relative;
z-index: 1;
background: red;
}
#content {
background: yellow;
width: 100%;
height: auto;
min-height: 300px;
}
#content .block {
position: relative;
background: #eee;
width: 90%;
top: -50px;
margin: 0 auto 0 auto;
box-sizing: border-box;
height: auto;
min-height: 400px;
z-index: 30;
}
<div id="sectionTitle">
...some text...
</div>
<div id="content">
<div class="block">
...element which should cover bottom of #sectionTitle element
</div>
</div>
1Why can't an element with a z-index value cover its child?
2What formatting context applies to elements that don't create their own?