How to use z-index in svg elements?

Specification

In the SVG specification version 1.1 the rendering order is based on the document order:

first element -> "painted" first

Reference to the SVG 1.1. Specification

3.3 Rendering Order

Elements in an SVG document fragment have an implicit drawing order, with the first elements in the SVG document fragment getting “painted” first. Subsequent elements are painted on top of previously painted elements.

Solution (cleaner-faster)

You should put the green circle as the latest object to be drawn. So swap the two elements.

<svg xmlns="http://www.w3.org/2000/svg" viewBox="30 70 160 120"> 
   <!-- First draw the orange circle -->
   <circle fill="orange" cx="100" cy="95" r="20"/> 

   <!-- Then draw the green circle over the current canvas -->
   <circle fill="green" cx="100" cy="105" r="20"/> 
</svg>

Here the fork of your jsFiddle.

Solution (alternative)

The tag use with the attribute xlink:href (just href for SVG 2) and as value the id of the element. Keep in mind that might not be the best solution even if the result seems fine. Having a bit of time, here the link of the specification SVG 1.1 “use” Element.

Purpose:

To avoid requiring authors to modify the referenced document to add an ID to the root element.

<svg xmlns="http://www.w3.org/2000/svg" viewBox="30 70 160 120">
    <!-- First draw the green circle -->
    <circle id="one" fill="green" cx="100" cy="105" r="20" />
    
    <!-- Then draw the orange circle over the current canvas -->
    <circle id="two" fill="orange" cx="100" cy="95" r="20" />
    
    <!-- Finally draw again the green circle over the current canvas -->
    <use xlink:href="#one"/>
</svg>

Notes on SVG 2

SVG 2 Specification is the next major release and still supports the above features.

3.4. Rendering order

Elements in SVG are positioned in three dimensions. In addition to their position on the x and y axis of the SVG viewport, SVG elements are also positioned on the z axis. The position on the z-axis defines the order that they are painted.

Along the z axis, elements are grouped into stacking contexts.

3.4.1. Establishing a stacking context in SVG

…

Stacking contexts are conceptual tools used to describe the order in which elements must be painted one on top of the other when the document is rendered, …

• SVG 2 Support Mozilla – Painting
• How do I know if my browser supports svg 2.0
• Can I use SVG
• Deprecated XLink namespace For SVG 2 use href instead of the additional deprecated namespace xlink:href (Thanks G07cha)