What is use of 'initial' value in CSS?

Source

The initial CSS keyword applies the initial value of a property to an element. It is allowed on every CSS property and causes the element for which it is specified to use the initial value of the property.

/* give headers a green border */
 h2 { border: medium solid green }

 /* but make those in the sidebar use the value of the "color" property */
 #sidebar h2 { border-color: initial; }
<p style="color:red"> 
    this text is red 
       <em style="color:initial"> 
          this text is in the initial color (e.g. black)
       </em>
    this is red again
 </p> 

The initial value (not attribute) denotes the initial value of the property, as defined in CSS specifications: “The ‘initial’ keyword represents the specified value that is designated as the property's initial value.” Thus, its meaning depends on the property, but not on anything else, e.g. not on the browser or on the element that the property is being applied to. So it does not mean browser default.

For example, for the display property, initial always means inline, because that’s the designated initial value of the property. In the example case, the browser default is block, since the element is div.

Thus, the initial value is of limited usefulness. Its main effect seems to be to confuse people, due to misunderstandings. A possible use case is for the color property, since its initial value is browser-dependent (mostly black, as we know, but not necessarily). For it, initial means browser default, since that’s how the property has been defined, A similar use case is for font-family: by declaring font-family: initial, you get browser’s default font (which may depend on browser settings).

The usefulness is further limited by lack of support on IE (even IE 10).