CSS Margins Tutorial

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1- CSS Margin

CSS Margin is used to create a space around an element, outside the element borders.
By standard syntax you need to provide 4 values for​​​​​​​ margin:
/* Syntax: */

margin: top right bottom left;

/* Example: */

margin: 5px 20px 30px 40px;
Instead of providing 4 values for CSS margin you can use 4 properties: CSS margin-top, margin-right, margin-bottom, margin-left.
margin-top: 5px;
margin-right: 20px;
margin-bottom: 30px;
margin-right: 40px;
CSS margin can also accept 1, 2 or 3 values
/* Apply to all four sides */
margin: 1em;
margin: -3px;

/* vertical | horizontal */
margin: 5px 10px;

/* top | horizontal | bottom */
margin: 10px 20px 30px;
Values can be given to  CSS margin:
auto Browser will automatically calculate  margin for element.
% Specify a value in% for margin, which is % relative to the width of the nearest ancestor Block-level Element containing the current element. See also the explanation at the end of this post. 
px, cm, em,.. Specify a specific value in px, cm, em,.. units ​​​​​​​, for example, 10px, 2cm, 3em,...
inherit The values of margin will be inherited from a parent element.

2- CSS Margin %

CSS margin accepts percentage value (%). This value is percentage relative to the width of the Containing Block, of which, the Containing Block is the nearest ancestor Block-level element that contains the current element. 
margin-percent-example.html
<!DOCTYPE html>
<html>
   <head>
      <title>CSS Padding</title>
      <meta charset="UTF-8"/>
      <script>
         function changeParentSize()  {
            var blockDiv = document.getElementById("blockDiv");

             // offsetWidth = contentWidth + padding + border
            var offsetWidth  = blockDiv.offsetWidth;
            if(offsetWidth > 300)  {
                offsetWidth = 200;
            }

            blockDiv.style.width = (offsetWidth+1) + "px";
         }

      </script>
   </head>

   <body>
      <h3>CSS Margin Percentage</h3>

      <div id="blockDiv" style="width:200px; height: 150px; border: 1px solid blue">
         I am a div (Block-Level Element) <br/>
         <span style="background-color: yellow;">
            I am a span (Inline Element)
            <div id= "redDiv" style="width:100px; margin:10%; border:1px solid red;">
               width:100px; margin:10%;
            </div>
         </span>
      </div>

      <br/>
      <button onClick="changeParentSize()">Change size of 'blockDiv'</button>
   </body>
</html>

3- CSS margin auto

CSS margin accepts auto value. This is a good solution when you want to  align elements in the middle horizontally.
margin-auto-example.html
<!DOCTYPE html>
<html>
   <head>
      <title>CSS Margin</title>
      <meta charset="UTF-8"/>
      <style>

        .center-div {
           width: 150px;
           background-color: LightCyan;
           padding:10px;
           margin-left: auto;
           margin-right: auto
        }

      </style>
   </head>

   <body>
      <h3>CSS Margin auto</h3>

      <div class="center-div">

          margin-left: auto; <br/>
          margin-right: auto;
      </div>
   </body>
</html>

4- Margin for Inline Element

Note: CSS margin vertically ( margin-top & margin-bottom) doesn't have effect on Inline Element. For example, by default <span> is an inline element, vertical margins have no effect on it.
margin-inline-element-example.html
<!DOCTYPE html>
<html>
   <head>
      <title>CSS Margin</title>
      <meta charset="UTF-8"/>

      <style>
         div {
            background-color: yellow;
         }
         span {
            margin-top: 100px;
            margin-bottom: 100px;
            margin-left: 20px;
            margin-right: 20px;
            background-color: #eee;
         }
      </style>

   </head>

   <body>
      <h3>CSS Margin for Inline Element</h3>

      <div>I am a div</div>
      <span>
         I am a span.
         margin-top: 100px;
         margin-bottom: 100px;
         margin-left: 20px;
         margin-right: 20px;
      </span>
      <div>I am a div</div>
      
   </body>
</html>
If you want vertical margin to work with an inline element, you have to turn it into a Block-Level Element or an Inline-block Element.
span  {
   display: block;
}

/* OR: */

span  {
   display: inline-block;
}

5- CSS margin collapsing

The bottom and top margins of two adjacent Block-level elements sometimes combine each other to form a single margin having the size which is the largest size of the two above margins. This behavior is referred to as margin collapsing. Note: margin collapsing does not occur with the elements that have  {float:left|right} or  {position:absolute}.
Margin collapsing with participation of many elements:
margin-collapsing-example.html
<!DOCTYPE html>
<html>
   <head>
      <title>CSS Margin Collapsing</title>
      <meta charset="UTF-8"/>
      <style>
         div {
           border:1px solid green;
         }
         button {
            margin-top:10px;
         }
      </style>

      <script>
         function addDiv2MarginTop(delta)  {
            var div2 = document.getElementById("div2");

            var style = window.getComputedStyle(div2);

            var marginTopString = style.marginTop;// 20px
            var marginTopInt = parseInt(marginTopString.substr(0,marginTopString.length-2)); // 20

            var newMarginTopString =  (marginTopInt + delta) + "px";

            div2.style.marginTop = newMarginTopString;
            div2.innerHTML = "height:50px; margin-top: " + newMarginTopString;
         }

      </script>
   </head>

   <body>
      <h3>CSS Margin Collapsing</h3>

      <div style="height:50px; margin-bottom:50px">
           height:50px; margin-bottom:50px;
      </div>

      <div style="height:50px; margin-top:40px" id="div2">
           height:50px; margin-top:40px
      </div>

      <button onClick="addDiv2MarginTop(3)">Div2 margin-top (+)</button>
      <button onClick="addDiv2MarginTop(-3)">Div2 margin-top (-)</button>


   </body>
</html>
If two block-level elements are adjacent, they will cause marginal collapsing (See the above example ). But if the second element is established CSS {float:left|right}, margin collapsing will not occur.
floating-example.html
<!DOCTYPE html>
<html>
   <head>
      <title>CSS Margin Collapsing</title>
      <meta charset="UTF-8"/>
      <style>
        div {
          border:1px solid green;
        }
         p {
             color:red;
             font-style:italic;
         }
      </style>
   </head>

   <body>
      <h3>CSS Margin Collapsing</h3>
      <p>Margin Collapsing does not work with {float:left|right}</p>

      <div style="height:50px; margin-bottom:50px;">
           height:50px; margin-bottom:50px;
      </div>

      <div style="height:50px; margin-top:40px; float:right" id="div2">
           height:50px; margin-top:40px; float:right;
      </div>  
   </body>
</html>

Parent element and child elements.

If the parent element has no border, padding, and the first child element is a Block-level element having no  {float:left|right}, margin collapsing will occur for the  margin-top of this child element and other element outside the parent element.
margin-collapsing-example2.html
<!DOCTYPE html>
<html>
   <head>
      <title>CSS Margin Collapsing</title>
      <meta charset="UTF-8"/>
      <style>
        div {
          background: lavender;
        }
        p {
          margin-top: 30px;
          margin-left: 15px;
          margin-right: 15px;
          background: yellow;
          height: 30px;
        }
      </style>
   </head>

   <body>
      <h3>CSS Margin Collapsing (Parent and First Child)</h3>

      <div style="margin-bottom:15px;">This is a div {margin-bottom: 15px;}</div>

      <div style="min-height:50px;">
        <p>
           Child Paragraph 1 - margin-top: 30px;
        </p>
        <p>
           Child Paragraph 2 - margin-top: 30px;
        </p>
        This parent element contains two paragraphs!
      </div>

   </body>
</html>
If the parent element has no border, padding, and the last child element is a Block-level element without  {float:left|right}, margin collapsing will occur with the  margin-bottom of this child element and other element outside the parent element.
margin-collapsing-example3.html
<!DOCTYPE html>
<html>
   <head>
      <title>CSS Margin Collapsing</title>
      <meta charset="UTF-8"/>
      <style>
        div {
          background: lavender;
        }

        p {
          margin-bottom: 30px;
          margin-left: 15px;
          margin-right: 15px;
          background: yellow;
          height: 30px;
        }
      </style>
   </head>

   <body>
      <h3>CSS Margin Collapsing (Parent and last Child)</h3>

      <div style="min-height:50px;">This parent element contains two paragraphs!
        <p>
           Child Paragraph 1 - margin-bottom: 30px;
        </p>
        <p>
           Child Paragraph 2 - margin-bottom: 30px;
        </p>
      </div>

      <div style="margin-top:15px;">This is a div {margin-top: 15px;}</div>

   </body>
</html>

Empty element.

If an element is an empty block without border, padding, height, min-height, then its  margin-top and  margin-bottom will collapse into a value.
empty-block-example.html
<!DOCTYPE html>
<html>
   <head>
      <title>CSS Margin Collapsing</title>
      <meta charset="UTF-8"/>

   </head>

   <body>
      <h3>CSS Margin Collapsing (Empty block)</h3>

      <div style="height:40px; background-color: yellow;">
           height:40px;
      </div>

      <div style="margin-top:40px; margin-bottom:50px;">
          <!-- This is Empty Block -->
          <!-- No border, padding, height, min-height -->
      </div>

      <div style="height:50px; background-color: yellow;">
           height:50px;
      </div>

   </body>
</html>

Note:

If a negative margin is involved in margin collapsing, then the margin value used will be the sum of the largest positive margin and the most negative margin.

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