Css: Margin-Top When Parent'S Got No Border

CSS: Margin-top when parent's got no border

You could add overflow:auto to .body to prevent margin-collapsing. See http://www.w3.org/TR/CSS2/box.html#collapsing-margins

CSS margin terror; Margin adds space outside parent element

Add overflow:auto to your #page div.

jsFiddle example

And check out collapsing margins while you're at it.

child looses margins if parent is without borders

Add padding in #logoc class and remove border from #wrapt class

#logoc {
    display: block;
    width: 50%;
    margin: 25px auto;
    padding: 10px;
}

Margin on child element moves parent element

Found an alternative at Child elements with margins within DIVs You can also add:

.parent { overflow: auto; }

or:

.parent { overflow: hidden; }

This prevents the margins to collapse. Border and padding do the same.
Hence, you can also use the following to prevent a top-margin collapse:

.parent {
    padding-top: 1px;
    margin-top: -1px;
}

2021 update: if you're willing to drop IE11 support you can also use the new CSS construct display: flow-root. See MDN Web Docs for the whole details on block formatting contexts.

Update by popular request:
The whole point of collapsing margins is handling textual content. For example:

h1, h2, p, ul {
  margin-top: 1em;
  margin-bottom: 1em;
  outline: 1px dashed blue;
}

div { outline: 1px solid red; }

<h1>Title!</h1>
<div class="text">
  <h2>Title!</h2>
  <p>Paragraph</p>
</div>
<div class="text">
  <h2>Title!</h2>
  <p>Paragraph</p>
  <ul>
    <li>list item</li>
  </ul>
</div>

Why would margin not be contained by parent element?

This is how CSS works according to W3C:

In this specification, the expression collapsing margins means that adjoining margins (no non-empty content, padding, or border areas, or clearance separate them) of two or more boxes (which may be next to one another or nested) combine to form a single margin.

More specific to your case of the top div:

If the top and bottom margins of a box are adjoining, then it is possible for margins to collapse through it. In this case, the position of the element depends on its relationship with the other elements whose margins are being collapsed.
If the element's margins are collapsed with its parent's top margin, the top border edge of the box is defined to be the same as the parent's.
Otherwise, either the element's parent is not taking part in the margin collapsing, or only the parent's bottom margin is involved. The position of the element's top border edge is the same as it would have been if the element had a non-zero bottom border.

The best thing I can do is point you to on "Collapsing Margins" on sitepoint (by Tommy Olsson and Paul O’Brien). They do a very detailed explanation with examples showing you exactly the behaviors you demoed in the question example code.

Css margin-top moves parent element

Explanation

Yes, it is a correct behaviour because the percents consider parents position. Like in your example, it will be positioned 45% below of body position.

But in your example, this wont work because 45% is not the center of any screen, see your example with some borders to see the behaviour:

body {  border:1px solid red;}
.custom{
  margin-left: 45%;  margin-top: 45%;    border:1px solid blue;  }

<div class="custom">      ...custom stuff here...   </div>

Collapsed margin in CSS

The margins of #empty collapse through, resulting in a 20px collapsed-through margin. This collapsed-through margin collapses with the 10px bottom margin of the first paragraph, and the 10px top margin of the last paragraph. This results in a 20px gap between the first and last paragraphs, since the collapsed-through margin is larger than either of their margins and therefore swallows them both.

Your observation is correct: #empty, when its collapsed through, is rendered with its top margin. From the spec:

[...] The position of the element's top border edge is the same as it would have been if the element had a non-zero bottom border.
Note that the positions of elements that have been collapsed through have no effect on the positions of the other elements with whose margins they are being collapsed; the top border edge position is only required for laying out descendants of these elements.

The position that "would have been if the element had a non-zero bottom border" is the position of the element if the element's margins did not collapse through, since having a non-zero bottom border blocks the margins from collapsing through.

Why do the top and bottom margins disappear in the child container?

In some cases, top and bottom margins are collapsed. You can read more about it here. What's happening is this:

If there is no border, padding, inline part, block formatting context created, or clearance to separate the margin-top of a block from the margin-top of its first child block, [...] then those margins collapse. The collapsed margin ends up outside the parent.

Why is the margin of my child html element outside the parent

It's "margin collapsing" which can seem confusing at first.

I recommend you read https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Box_Model/Mastering_margin_collapsing

Css: Margin-Top When Parent'S Got No Border