CSS - Rounded concave corners for compound layout - html

Question

First, I know there are similar questions available (like. Create concave corners in css) but they don't really cover this situation.
This is not about single cell/div element.
I have three blocks that will have some text content inside:
top-middle centered block (narrow)
middle-middle block (screen-wide)
bottom-middle centered block (narrow)
Basically something like a cross (text removed):
The outer corners (8) is straighforward but how could I achieve those inner ones (4)?

Answer 1

see bellow code, maybe it needs some adjustments but the idea is that you use pseudo-elements to make those inner borders
let me know if this is what you want
.colored {
background:yellow;
border:5px solid green;
width:100px;
height:100px;
box-sizing:border-box;
position:relative;
}
#content {
width:300px;
position:relative;
background:#000;
}
.top,.bottom { position:relative;margin:0 auto;clear:both}
.top { border-bottom:none}
.bottom { border-top:none}
.left { border-right:none}
.right { border-left:none;}
.colored.center { border:none;}
.left,.center,.right { float:left;}
.top { border-top-left-radius:10px;border-top-right-radius:10px;}
.bottom { border-bottom-left-radius:10px;border-bottom-right-radius:10px;}
.right { border-bottom-right-radius:10px;border-top-right-radius:10px;}
.left { border-bottom-left-radius:10px;border-top-left-radius:10px;}
.top:before {
position:absolute;
height:100%;
width:100%;
left:-100%;
top:5px;
content:"";
border-bottom-right-radius:10px;
border-right:5px solid green;
border-bottom:5px solid green;
z-index:9999;
box-sizing:border-box;
}
.top:after {
position:absolute;
height:100%;
width:100%;
right:-100%;
top:5px;
content:"";
border-bottom-left-radius:10px;
border-left:5px solid green;
border-bottom:5px solid green;
z-index:9999;
box-sizing:border-box;
}
.bottom:before {
position:absolute;
height:100%;
width:100%;
left:-100%;
bottom:5px;
content:"";
border-top-right-radius:10px;
border-right:5px solid green;
border-top:5px solid green;
z-index:9999;
box-sizing:border-box;
}
.bottom:after {
position:absolute;
height:100%;
width:100%;
right:-100%;
bottom:5px;
content:"";
border-top-left-radius:10px;
border-left:5px solid green;
border-top:5px solid green;
z-index:9999;
box-sizing:border-box;
}
<div id="content">
<div class="top colored">
</div>
<div class="left colored">
</div>
<div class="center colored">
</div>
<div class="right colored">
</div>
<div class="bottom colored">
</div>
</div>

Answer 2

Variation with just three divs, a bit hacky but functional. Uses pseudo elements, transforms and inner box-shadow.
div {
background-color: #000;
min-height: 100px;
}
.center {
width: 100px;
margin: 0 auto;
}
.rounded {
border-radius: 20px;
border: 5px solid red;
}
.conc {
position: relative;
}
.conc::before,
.conc::after {
content: '';
position: absolute;
border: 5px solid red;
border-radius: 20px;
width: 25px;
height: 25px;
background-color: trnaspanret;
border-color: red transparent transparent;
z-index: 3;
box-shadow: white 0px 0px 0px 20px inset
}
.conc.bottom {
margin-bottom: -5px;
border-bottom: 0;
border-radius: 20px 20px 0 0
}
.conc.top {
margin-top: -5px;
border-top: 0;
border-radius: 0 0 20px 20px
}
.conc::before {
left: -35px;
}
.conc::after {
right: -35px;
}
.conc.top::before,
.conc.top::after {
top: 0px;
}
.conc.bottom::before,
.conc.bottom::after {
bottom: 0px;
}
.conc.bottom::before {
transform: rotate(135deg)
}
.conc.bottom::after {
transform: rotate(-135deg)
}
.conc.top::before {
transform: rotate(45deg)
}
.conc.top::after {
transform: rotate(-45deg)
}
.centerblinders {
position: relative;
}
.centerblinders::before,
.centerblinders::after {
content: '';
position: absolute;
width: 130px;
height: 20px;
background-color: #000;
left: 50%;
transform: translatex(-50%);
z-index: 2;
}
.centerblinders::before {
top: -15px;
}
.centerblinders::after {
bottom: -15px;
}
<div class="rounded center conc bottom"></div>
<div class="rounded centerblinders"></div>
<div class="rounded center conc top"></div>