How come adjusting the margin of a div doesn't impact the placement of its child elements?

Question

How come adjusting the margin of a div doesn't impact the placement of its child elements?

I'm trying to wrap my head around how css margins interact with divs and their child elements.

For instance, when I set it up like this...

<div style="clear: both; margin-top: 2em;">
    <input type="submit" value="Save" />
</div>

...the Save button ends up right next to the User Role (margin fail):

Margin Fail :( http://img697.imageshack.us/img697/8459/nomargin.jpg

But changing it to this setup...

<div style="clear: both;">
    <input style="margin-top: 2em;" type="submit" value="Save" />
</div>

...creates a gap between the Save button and the User Role (margin win):

Margin Win :) http://img20.imageshack.us/img20/1973/yesmargin.jpg

Questions:

Can someone help me understand why the div's margin doesn't push the input element down? Why is it necessary to apply the margin directly to the input? Is there a core principle of CSS that I am missing here?

html css

Answer 1

Answer №1

The reason for this behavior is that the div element doesn't have a neighboring element to create space between itself and. It appears that the preceding select element is floated, causing it to be removed from the normal document flow and losing its influence on margin calculations. As a result, the div clears the float by moving below it and checking for a 2em margin to the nearest element above it within the same flow. In this case, there is such a margin, so no further movement occurs.

In contrast, setting the margin on the submit button is straightforward because its reference point is the parent div.

Answer 2

The reason for this behavior is that the div element doesn't have a neighboring element to create space between itself and. It appears that the preceding select element is floated, causing it to be removed from the normal document flow and losing its influence on margin calculations. As a result, the div clears the float by moving below it and checking for a 2em margin to the nearest element above it within the same flow. In this case, there is such a margin, so no further movement occurs.

In contrast, setting the margin on the submit button is straightforward because its reference point is the parent div.

Answer 3

Answer №2

When you apply a margin to an element, it only affects that specific element's margin. It does not get inherited by other elements. If you are expecting inheritance, consider using padding instead. Here's an example:

<div style="clear: both; padding-top: 2em;">
  <input type="submit" value="Save" />
</div>

Refer to the Box Model for more information:

Additionally, keep in mind that margins can merge. For instance:

<div>one</div>
<div>two</div>

with:

div { margin: 1em; }

will result in a 1em gap between them, not 2em. Check out 8.3.1 Collapsing Margins:

Vertical margins may collapse between certain boxes:

Two or more adjoining vertical margins of block boxes in the normal flow collapse. The resulting margin width is the maximum of the adjoining margin widths. In cases of negative margins, the adjustment is calculated differently. Make sure to refer to the complete explanation.

If the element before your div already has a margin at the bottom, adding a margin to your div might not push it down as expected. Visualizing with borders can help understand this better.

In conclusion, experiment with border settings to visualize how margins are affecting your layout.

Answer 4

When you apply a margin to an element, it only affects that specific element's margin. It does not get inherited by other elements. If you are expecting inheritance, consider using padding instead. Here's an example:

<div style="clear: both; padding-top: 2em;">
  <input type="submit" value="Save" />
</div>

Refer to the Box Model for more information:

Additionally, keep in mind that margins can merge. For instance:

<div>one</div>
<div>two</div>

with:

div { margin: 1em; }

will result in a 1em gap between them, not 2em. Check out 8.3.1 Collapsing Margins:

Vertical margins may collapse between certain boxes:

Two or more adjoining vertical margins of block boxes in the normal flow collapse. The resulting margin width is the maximum of the adjoining margin widths. In cases of negative margins, the adjustment is calculated differently. Make sure to refer to the complete explanation.

If the element before your div already has a margin at the bottom, adding a margin to your div might not push it down as expected. Visualizing with borders can help understand this better.

In conclusion, experiment with border settings to visualize how margins are affecting your layout.

Answer 5

Answer №3

In the initial scenario, we observe that the floated items positioned above the button are drifting outside of their container.

Although the margins seem to be functioning properly, they are being applied between the floating components and the button's parent element instead of directly between the floaters and the button itself. Due to the absence of non-floating elements within the parent container, its height is reduced to zero, causing the floaters to intersect with the margins.

Answer 6

In the initial scenario, we observe that the floated items positioned above the button are drifting outside of their container.

Although the margins seem to be functioning properly, they are being applied between the floating components and the button's parent element instead of directly between the floaters and the button itself. Due to the absence of non-floating elements within the parent container, its height is reduced to zero, causing the floaters to intersect with the margins.

Answer 7

Answer №4

When looking at your initial example, it appears that you are applying a margin directly to the DIV itself. Visualize the DIV as a container that holds all nested elements within it. In this scenario, the nested element is the button. If you structure it like this:

<div style="clear: both; margin-top: 2em;">
    <input type="submit" value="Save" />
</div>

The style change does not impact the child elements' styling. Although the button's position may shift relatively, the actual style of the child element remains unchanged. Now, consider the alternative:

<div style="clear: both; margin-top: 2em;">
    <input type="submit" value="Save" />
</div>

In this case, you are adjusting the margin of the child element. As a result, the outcome will be different. Modifying the child element's margin will not influence the parent element (DIV).

Answer 8

When looking at your initial example, it appears that you are applying a margin directly to the DIV itself. Visualize the DIV as a container that holds all nested elements within it. In this scenario, the nested element is the button. If you structure it like this:

<div style="clear: both; margin-top: 2em;">
    <input type="submit" value="Save" />
</div>

The style change does not impact the child elements' styling. Although the button's position may shift relatively, the actual style of the child element remains unchanged. Now, consider the alternative:

<div style="clear: both; margin-top: 2em;">
    <input type="submit" value="Save" />
</div>

In this case, you are adjusting the margin of the child element. As a result, the outcome will be different. Modifying the child element's margin will not influence the parent element (DIV).

How come adjusting the margin of a div doesn't impact the placement of its child elements?

Answer №1

Answer №2

Answer №3

Answer №4

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