PNG shadow with fluid height - css

Question

Due to browser performance implications I can't use box-shadow CSS property because I have many similarly looking elements on my page that should have same looking style including shadow. That's the reason I would like to implement shadows using traditional PNG imagery.
Facts
My elements have predefined and more importantly fixed pixel width
They have fluid height (auto) depending on their content
They have content directly in the element and some child elements will be positioned outside their border
CSS3 can be used but performance-critical parts (gradients, shadows...) should be avoided
CSS pseudo elements can be used without limitation
Requirements
There should be no additional wrapper element added in order to have fluid shadow
Application should run smoothly on mobile browsers - shadows seem to slow down performance significantly on mobile devices since their processing power is much lower than desktop computers.
Possible direction
I thought of using :before and :after pseudos to display top-to-bottom and bottom shadows on the containing element, but these pseudos display within their parent element and positioning parent z-index higher than these children has no effect.
Visual demo of end result
This JSFiddle Demo in pure CSS3 that I would like to achieve but using PNG shadows. In reality there are numerous of these boxes so you can imagine mobile browsers are struggling with all these shadows.
Item is one such box (see blow) that needs PNG shadow. Left menu is child element positioned outside of the box.
Display in Chrome
HTML
<div class="item">
<menu>
<li>Yes</li>
<li>No</li>
<li>Maybe</li>
</menu>
<div class="content">
Some content
</div>
</div>
CSS3 LESS
.item {
position: relative;
width: 300px;
background-color: #fff;
box-shadow: 0 0 10px #ccc;
margin: 20px 20px 20px calc(20px + 3.5em);
min-height: 5em;
&:first-child {
margin-top: 0;
}
&:after {
content: "";
position: absolute;
top: 0;
left: 0;
width: 10px;
height: 5em;
background-color: #fff;
}
menu {
position: absolute;
top: 0;
left: -3.5em;
width: 3.5em;
margin: 0;
border: 0;
padding: 0;
list-style: none;
background-color: #fff;
box-shadow: 0 0 10px #ccc;
li a {
display: block;
text-align: center;
padding: 2px 0;
}
}
.content {
padding: .75em 1em;
}
}

Answer 1

Probably I am missing something, but looks like you want something in this way:
demo
The CSS is
.base {
width: 300px;
height: 150px;
font-size: 100px;
font-weight: bolder;
background-color: lightgreen;
position: relative;
z-index: auto;
}
.base:after {
content: '';
position: absolute;
top: 30px;
left: 30px;
background-color: green;
z-index: -1;
width: 100%;
height: 100%;
}
.child {
position: absolute;
left: 150px;
top: 50px;
border: solid 1px black;
color: red;
}
And just change the background of the :after to your image.
I have applied this solution to your fiddle.
The relevant CSS is for the before pseudo element:
.item:before {
content: "";
position: absolute;
top: -10px;
left: -10px;
right: -10px;
bottom: -10px;
z-index: -1;
background-image: url(http://placekitten.com/100/100);
background-repeat: no-repeat;
background-size: 100% 100%;
}
I have used a kitten picture, that is being scaled to cover all the needed size. Just change that to whatever you want.
I needed to do it that way because I had onky a pseudo element available.
The key for that to work (and where you probably had the difficulty) is to add z-index: auto to .item
Updated demo
Well, I had said that it wasn't posible, but I have find a way.
The standard technique would be to use 2 elements, just to avoid stretching the image (as you said). The problem is that we only have 1 pseudo element available.
The solution then would be to use 1 pseudo element, but with 2 backgrounds, to solve the issue.
CSS (only relevant part)
.item:before {
background-image: url(http://placekitten.com/320/10), url(http://placekitten.com/320/500);
background-repeat: no-repeat;
background-size: 100% 9px, 100% calc(100% - 9px);
background-position: left bottom, left top;
}
We will need an image (the first one) only 10 px in height, to cover the bottom shadow. And another one, with enough height to cover the maximumitem posible, and that will be used for the remaining part of the shadow. The dark part is that we need now a calc() height, with limited support. (anyway, better than border image)
demo 3