Why Is an Element with Position: Fixed Moving with a Non-Positioned Sibling

html {    background-color: green;    height: 100%; }
body {    background-color: pink;    height: 100%;}
header {   position: fixed;   border: 1px solid red;}
main {  margin-top: 90px;  background-color:yellow;}

<header>Project Header</header> <main class="container" id="layout-mainContent">    <div class="row" id="first-row">somecontent</div></main>

position fixed div in absolute positioned div works - but why?

An element with position: fixed is indeed positioned relative to the viewport (or browser). However, because it is an absolutely positioned element, it is "positioned relative to the initial containing block established by the viewport".

This is laid out in the position documentation:

An absolutely positioned element is an element whose computed position value is absolute or fixed. The top, right, bottom, and left properties specify offsets from the edges of the element's containing block. (The containing block is the ancestor relative to which the element is positioned.) If the element has margins, they are added to the offset.

That is to say, when you specify margin-top and margin-left, these values are relative to the parent. And because the element is positioned relative to the parent, the default top and left are inherited from the parent. In your example, the .fixed element has a top value of 100px because it inherits the top value of 100px from .sidebar (the parent). When you set top: 0 on .fixed, you are overriding this value (going from top: 108px to top: 0):

Because of this, the element appears to be taken 'out of flow'. However, it is still always positioned relative to the viewport; it just had an initial offset (which it inherited from its parent).

Fixed position but relative to container

Short answer: no. (It is now possible with CSS transform. See the edit below)

Long answer: The problem with using "fixed" positioning is that it takes the element out of flow. thus it can't be re-positioned relative to its parent because it's as if it didn't have one. If, however, the container is of a fixed, known width, you can use something like:

#fixedContainer {
  position: fixed;
  width: 600px;
  height: 200px;
  left: 50%;
  top: 0%;
  margin-left: -300px; /*half the width*/
}

http://jsfiddle.net/HFjU6/1/

Edit (03/2015):

This is outdated information. It is now possible to center content of an dynamic size (horizontally and vertically) with the help of the magic of CSS3 transform. The same principle applies, but instead of using margin to offset your container, you can use translateX(-50%). This doesn't work with the above margin trick because you don't know how much to offset it unless the width is fixed and you can't use relative values (like 50%) because it will be relative to the parent and not the element it's applied to. transform behaves differently. Its values are relative to the element they are applied to. Thus, 50% for transform means half the width of the element, while 50% for margin is half of the parent's width. This is an IE9+ solution

Using similar code to the above example, I recreated the same scenario using completely dynamic width and height:

.fixedContainer {
    background-color:#ddd;
    position: fixed;
    padding: 2em;
    left: 50%;
    top: 0%;
    transform: translateX(-50%);
}

If you want it to be centered, you can do that too:

.fixedContainer {
    background-color:#ddd;
    position: fixed;
    padding: 2em;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
}

Demos:

jsFiddle: Centered horizontally only

jsFiddle: Centered both horizontally and vertically

Original credit goes to user aaronk6 for pointing it out to me in this answer

Topmost 'fixed' position div moving with non position div

Change the CSS of the your .bottom element to:

.bottom {
    position:relative;
    top:128px; #64+64
}

Why absolute positioned element looks at its sibling element?

position:absolute; looks for the nearest element with position:relative/absolute, and makes his position, based on that element.

In your case, as you have position:relative in the image, the "Irene" text will be placed at the top left corner of the image.

For "Irene" to be at the top left of the document, you either apply position:fixed, wich is based on the browser window, or remove all the position:absolute/relative that are above the "Irene" element.

Why is an absolutely positioned element placed by its sibling instead of at the top corner of the page?

I always thought that absolute positioned elements are out of the flow.

Yes, they are removed from normal flow.

I don't understand why absolute positioned element appeared after child_static div.

Just because absolute positioning removes elements from normal flow, it doesn't mean it does alter the position of the elements as well.

In other words, absolutely positioned elements would be at the same place as they are not positioned absolutely unless their top, left, ... offsets are set.

So what happens is that they would overlap next sibling elements, because they are not part of document flow anymore.

For instance have a look at the following example where the gold <div> element is covered by absolutely positioned element.

.parent {    position: relative;}
.child_static {    height: 20px;    background: blue;}
.child_absolute {    position: absolute;    height: 20px;    width: 100%;    background: green;}
.child_static ~ .child_static {    background: gold;}

<div class='parent'>    <div class='child_static'>Green</div>    <div class='child_absolute'>Blue</div>    <div class='child_static'>Gold</div></div>

#parentDiv { position:relative; }
#childDiv { position:absolute; left:50px; top:20px; }

#yourDiv { position:fixed; bottom:40px; right:40px; }