What Are the Default Top, Left, Botton or Right Values When Position:Absolute Is Used

What are the default top, left, botton or right values when position:absolute is used?

As you suspect, the initial values for all these properties is not 0; it is auto. You can find their property definitions in section 9.3.2 of the spec.

When an absolutely positioned box keeps all its offsets auto (i.e. you don't modify them), it doesn't go anywhere. It remains in the static position, which basically means its usual spot in the layout had it not been positioned at all. Section 10 has all the details (it even has entire paragraphs explaining what "static position" means), but you'll want to focus on 10.3.7:

The constraint that determines the used values for these elements is:

'left' + 'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' + 'right' = width of containing block

If all three of 'left', 'width', and 'right' are 'auto': First set any 'auto' values for 'margin-left' and 'margin-right' to 0. Then, if the 'direction' property of the element establishing the static-position containing block is 'ltr' set 'left' to the static position and apply rule number three below; otherwise, set 'right' to the static position and apply rule number one below.

[...]

1. 'left' and 'width' are 'auto' and 'right' is not 'auto', then the width is shrink-to-fit. Then solve for 'left'

And 10.6.4:

For absolutely positioned elements, the used values of the vertical dimensions must satisfy this constraint:

'top' + 'margin-top' + 'border-top-width' + 'padding-top' + 'height' + 'padding-bottom' + 'border-bottom-width' + 'margin-bottom' + 'bottom' = height of containing block

If all three of 'top', 'height', and 'bottom' are auto, set 'top' to the static position and apply rule number three below.

[...]

3. 'height' and 'bottom' are 'auto' and 'top' is not 'auto', then the height is based on the content per 10.6.7, set 'auto' values for 'margin-top' and 'margin-bottom' to 0, and solve for 'bottom'

Default value for top, bottom, right, left position properties

Take top for example, from https://developer.mozilla.org/en/docs/Web/CSS/top

For absolutely positioned elements (those with position: absolute or position: fixed), it specifies the distance between the top margin edge of the element and the top edge of its containing block.

So, top: auto for an absolutely positioned element means whatever the position of the element currently is, and can be effected by the bottom property.

For relatively positioned elements (those with position: relative), it
specifies the amount the element is moved below its normal position.

So, top: auto for a relative positioned element it means the offset from it's original position, also based on the bottom property. If both are set to auto, then it won't offset anything

This would be the same for bottom, but opposite, and then similarly for the left/right relationship.

From http://vanseodesign.com/css/auto-positioning/

The default value for the top, right, bottom and left properties is
auto, which means the absolutely positioned box will appear exactly
where it would have had if it wasn’t positioned. Since it’s removed
from the flow it will overlap any elements in the normal flow that
follow it, though.

Also of note is this:

When both top and bottom are specified, as long as height is
unspecified, auto or 100%, both top and bottom distances will be
respected. Otherwise, if height is constrained in any way, the top
property takes precedence and the bottom property is ignored.

position: absolute without setting top/left/bottom/right?

The standard generally says if top/bottom, left/right are auto, then default them to their position: static values:

http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width

http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height

Position:absolute without top/left/bottom/right values

If you want an element to remain in its static position (where it would normally be if it were not positioned) but simply take it out of normal flow, simply specifying position: absolute is perfectly acceptable. The behavior is as described in sections 10.3.7 and 10.6.4 of the spec, and every browser behaves correctly.

You aren't required to give it any offsets if you don't want to move the element from its usual static position, and you aren't required to relatively position its parent element if you're not going to move the element anywhere since it'll remain in its static position anyway.

I just looked at your code again and noticed that you're applying percentage padding to your absposed element. You will need to relatively position the parent element if you want the percentage padding to be calculated based on the width of this parent element and not the initial containing block (where the root element resides):

#parent{
    position:relative;
    width:50%;
    margin:10% auto;
    background:gold;
    height:20%;
}

Compare this fiddle with the original.

So, the main purpose of relatively positioning some ancestor of an absolutely-positioned element is for designating its containing block. Sections 9 and 10 have most of the gory details on the interactions between elements and their containing blocks. Depending on your layout you may not need to position anything else at all, however if your layout is complex enough you may find that there are side-effects resulting from whether or not you position any container elements, and which one you do position. I suspect the topic of containing blocks could be covered in a different question since it may very well be out of scope of this one.

What are the left and top values of an element and its child element if both have position:absolute

First, you should note that the initial values for top, left, right and bottom are auto and not 0. See section 9.3.2 of the specs

So to the question:

if I didn't specify the left & top for absolutely positioned
element whose parent is also absolutely positioned & has no margin or
padding, the left and top would be 0px, Is that correct as per css
specs?

The answer is no this isn't correct. In your example, it happens to be true because the child element is positioned there in the flow of the document (even without any positining).

As you can see in this example :

<div style="position:absolute; width:100px; height:100px;">  Some text  <span style="position:absolute;">Test child</span></div>

top:0;
left:0;
height:100%;
width:100%;

input,div {  position: absolute;  top: 0;  left: 0;  bottom: 0;  right: 0;  text-align: center;  line-height: 2.2em;  margin: auto;}
div {  background: lime;/* show me */}
input {  z-index: 1;/* let me be on top */}

<input type="text" value="i have a defaut size" /><div> well i have size that content, coordonates or height/width ,  shall give me </div>