CSS Padding Tutorial

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1- CSS Padding

CSS Padding property is used to create a space surrounding the content of element and located in the element border like the following illustration: 
You can provide 4 values for​​​​​​​  CSS padding:
/* Syntax: */

padding: padding-top padding-right padding-bottom padding-left;

/* Example: */

padding:  5px 20px 30px 40px;
Instead of providing 4 values for CSS padding you can use 4 properties: CSS padding-top, padding-right, padding-bottom, padding-left.
padding-top: 5px;
padding-right: 20px;
padding-bottom: 30px;
padding-right: 40px;
If you provide 2 values for CSS padding, the first value is assigned to padding-top & padding-bottom, and the second value is assigned to​​​​​​​ padding-right & padding-left.
If you provide 3 values for  CSS padding, the first value is assigned to  padding-top, the second value is assigned to padding-right & padding-left, and the third value is assigned to​​​​​​​ padding-bottom.
CSS padding accepts specific values in px, pt, cm, ... or percent ( % ).
/* Apply to all four sides */
padding: 1em;

/* vertical | horizontal */
padding: 5% 10%;

/* top | horizontal | bottom */
padding: 1em 2em 2em;

/* top | right | bottom | left */
padding: 5px 1em 0 2em;

/* Global values */
padding: inherit;
padding: initial;
padding: unset;
 

2- Padding makes an element bigger

The presence of CSS padding makes an element larger than itself without establishing  CSS padding.
Please see the following example:
padding-and-width-height.html
<!DOCTYPE html>
<html>

<head>
    <title>CSS Padding</title>
    <meta charset="UTF-8"/>
</head>

<body>

    <h3>CSS Padding</h3>


    <div style="width:200px; height:50px; border: 1px solid blue;">
        width:200px; height:80px;
    </div>

    <br/>

    <div style="width:200px; height:50px; padding: 20px; border: 1px solid red;">
        width:200px; height:80px; padding: 20px;
    </div>


</body>
</html>
The actual size of the element you see is calculated by the following formula:
realWidth = contentWidth + cssBorderWidthLeft + cssBorderWidthRight + cssPaddingLeft + cssPaddingRight

realHeight = contentHeight + cssBorderWidthTop + cssBorderWidthBottom + cssPaddingTop + cssPaddingBottom
In case, by default contentWidth = cssWidth and  contentHeight = cssHeight.
// By Default:

contentWidth = cssWidth
contentHeight = cssHeight
Using CSS box-sizing:border-box will not increase element size when you use  CSS padding.

3- Padding with percentage value

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

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

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

      </script>
   </head>

   <body>
      <h3>CSS Padding Percentage</h3>

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

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