z-index not working with fixed positioning

I have a div with default positioning (i.e. position:static) and a div with a fixed position.

If I set the z-indexes of the elements, it seems impossible to make the fixed element go behind the static element.

    #over {
      width: 600px;
      z-index: 10;
    }
    
    #under {
      position: fixed;
      top: 5px;
      width: 420px;
      left: 20px;
      border: 1px solid;
      height: 10%;
      background: #fff;
      z-index: 1;
    }

    <!DOCTYPE html>
    <html>
       <body>
          <div id="over">
             Hello Hello HelloHelloHelloHelloHello Hello Hello Hello Hello Hello Hello Hello Hello Hello Hello
          </div>  
          <div id="under">
          </div>
       </body>
    </html>

Or on jsfiddle here: http://jsfiddle.net/mhFxf/

I can work around this by using position:absolute on the static element, but can anyone tell me why this is happening?

(There seems to be a similar question to this one, (Fixed Positioning breaking z-index) but it doesn't have a satisfactory answer, hence I am asking this here with my example code)

Add position: relative; to #over

    #over {
      width: 600px;
      z-index: 10;
      position: relative;    
    }
    
    #under {
      position: fixed;
      top: 5px;
      width: 420px;
      left: 20px;
      border: 1px solid;
      height: 10%;
      background: #f0f;
      z-index: 1;
    }

    <!DOCTYPE html>
    <html>
    <body>
        <div id="over">
            Hello Hello HelloHelloHelloHelloHello Hello Hello Hello Hello Hello Hello Hello Hello Hello Hello
        </div>  
    
        <div id="under"></div>
    </body>
    </html>

Fiddle

This question can be solved in a number of ways, but really, knowing the stacking rules allows you to find the best answer that works for you.

Solutions

The <html> element is your only stacking context, so just follow the stacking rules inside a stacking context and you will see that elements are stacked in this order

The stacking context’s root element (the <html> element in this case)

Positioned elements (and their children) with negative z-index values (higher values are stacked in front of lower values; elements with the same value are stacked according to appearance in the HTML)

Non-positioned elements (ordered by appearance in the HTML)

Positioned elements (and their children) with a z-index value of auto (ordered by appearance in the HTML)

Positioned elements (and their children) with positive z-index values (higher values are stacked in front of lower values; elements with the same value are stacked according to appearance in the HTML)

So you can

set a z-index of -1, for #under positioned -ve z-index appear behind non-positioned #over element
set the position of #over to relative so that rule 5 applies to it

The Real Problem

Developers should know the following before trying to change the stacking order of elements.

When a stacking context is formed
- By default, the <html> element is the root element and is the first stacking context
Stacking order within a stacking context

The Stacking order and stacking context rules below are from this link

When a stacking context is formed

When an element is the root element of a document (the <html> element)
When an element has a position value other than static and a z-index value other than auto
When an element has an opacity value less than 1
Several newer CSS properties also create stacking contexts. These include: transforms, filters, css-regions, paged media, and possibly others. https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index/The_stacking_context
As a general rule, it seems that if a CSS property requires rendering in an offscreen context, it must create a new stacking context.

Stacking Order within a Stacking Context

The order of elements:

The stacking context’s root element (the <html> element is the only stacking context by default, but any element can be a root element for a stacking context, see rules above)
- You cannot put a child element behind a root stacking context element
Positioned elements (and their children) with negative z-index values (higher values are stacked in front of lower values; elements with the same value are stacked according to appearance in the HTML)
Non-positioned elements (ordered by appearance in the HTML)
Positioned elements (and their children) with a z-index value of auto (ordered by appearance in the HTML)
Positioned elements (and their children) with positive z-index values (higher values are stacked in front of lower values; elements with the same value are stacked according to appearance in the HTML)

since your over div doesn't have a positioning, the z-index doesn't know where and how to position it (and with respect to what?). Just change your over div's position to relative, so there is no side effects on that div and then the under div will obey to your will.

here is your example on jsfiddle: Fiddle

edit: I see someone already mentioned this answer!

z-index only works within a particular context i.e. relative, fixed or absolute position.

z-index for a relative div has nothing to do with the z-index of an absolutely or fixed div.

EDIT ~~This is an incomplete answer.~~ This answer provides false information. Please review @Dansingerman's comment and example below.

z-index not working with fixed positioning

Solutions

The Real Problem

When a stacking context is formed

Stacking Order within a Stacking Context

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