What happens when nesting elements with position: fixed inside each other?

Question

The fixing and the positioning are two separate things. They're positioned the same as absolutely positioned elements: relative to their containing block. But in contrast with absolutely positioned elements, they remain fixed to that position with respect to the viewport (i.e. they don't move when scrolling):

http://www.w3.org/TR/CSS2/visuren.html#propdef-position

The box's position is calculated according to the 'absolute' model, but in addition, the box is fixed with respect to some reference.

Positioning

The definition of containing block says:

If the element has 'position: fixed', the containing block is established by the viewport in the case of continuous media (...)

and

If the element has 'position: absolute', the containing block is established by the nearest ancestor with a 'position' of 'absolute', 'relative' or 'fixed' (...)

which suggests that while their positioning algorithm is the same (they're both positioned relative to their containing block), the containing block for fixed elements is always the viewport, in contrast with absolutely positioned elements, so they should be positioned relative to that and not to any absolutely or fixed-positioned elements.

And as a matter of fact, that is indeed the case. For example, if you add top: 20px to .fixed, both divs will be positioned 20 pixels from the top of the viewport. The nested fixed div does not get positioned 20 pixels down from the top of its parent.

The reason you're not seeing that in this case is because you're not actually setting any of the left/top/right/bottom properties, so their positions are determined by the position they would have in the flow (their "static position"), which as my first quote said, is done according to the absolute model.

First element

position: fixed;

And the insider element must be:

position: sticky;

This only occurs when one of the ancestors of an element that uses position: fixed has defined (with a value different that none) any of the following styling rules: filter, transform, perspective, where, in this case, the closest ancestor that matches this condition will be used as the reference for the coordinates of the element instead of the viewport.

From MDN

fixed

The element is removed from the normal document flow, and no space is created for the element in the page layout. It is positioned relative to the initial containing block established by the viewport, except when one of its ancestors has a transform, perspective, or filter property set to something other than none (see the CSS Transforms Spec), in which case that ancestor behaves as the containing block. (Note that there are browser inconsistencies with perspective and filter contributing to containing block formation.) Its final position is determined by the values of top, right, bottom, and left.

This value always creates a new stacking context. In printed documents, the element is placed in the same position on every page.

I don't think this is really the intent. Things with fixed positioning are all positioned in relation to the window, if you have a fixed a child of another fixed, what do you want to happen? You can easily duplicate the behavior by not just position both of the fixed elements separately, or using other position to alter the child's position within the fixed element. :D

I dont think there is anything more to this then what w3c say there is:

Generates an absolutely positioned element, positioned relative to the browser window. The element's position is specified with the "left", "top", "right", and "bottom" properties

So if you get rid of that "padding: 40px;" you will get 2 elements - 1 over another.

Same effect like if you positioned both elements absolutely to top:0 left:0 with same parent(body).