How to Reset a CSS Variable (Aka Custom Properties) and Use the Fallback One

* {  box-sizing: border-box;}
#ex {  background: yellow;  padding: 0.5rem;  margin: 1rem 0;}

#div1 {  --unset-it: 2px dotted red;  outline: var(--unset-it, unset);}

<div id="ex">  <p id="div1">    outline: 2px dotted red;  </p>  <p id="div1" style="--unset-it:initial">    outline: unset;  </p></div>

css variable - default value on var not getting passed

Custom properties inherit.

From the spec:

Custom properties are ordinary properties, so they can be declared on
any element, are resolved with the normal inheritance and cascade
rules, ...

This means that if no value is set for a custom property on a given element, the value of its parent is used.

In this case, the parent element defines the custom property: --span:

<div class="item" style="--span: 7">

So the value of the --span property will be 7 on all the children - unless a different value is explicitly defined on a child.

Now the difference between the two versions of the code is clear.

In the commented version, var(--span, 1) in the .item class evaluates to 7 due to inheritance

Since there are only 2 columns in the grid, each item spans the full grid width

In the uncommented version (with --span: 1; defined in the .item class), var(--span, 1) in the .item class evaluates to 1 - because it is explicitly defined on the child.

NB

Fallback values are only used as the substitution value when a given custom property is invalid / not defined

In this case, the second item in the outer grid defines the --span custom property with value 7 and it is available to all its child elements.

How to store inherit value inside a CSS variable (aka custom property)?

In such case, we can consider the fallback value of a CSS variable. Like explained in the specification we can write something like this:

background:var(--color,inherit)

By doing this, we tell our property (background) to use inherit in case --color is not defined.

This may solve the issue but in our case it won't be enough since --color is always defined at :root level and will get inherited¹ by the pseudo element thus we will never use the fallback value.

To fix this we can consider the initial value in order to undefine our custom property and force the use of the fallback value. As described in the specification:

The initial value of a custom property is an empty value; that is, nothing at all. This initial value has a special interaction with the var() notation, which is explained in the section defining var().

and

To substitute a var() in a property’s value:

1. If the custom property named by the first argument to the var()
   function is animation-tainted, and the var() function is being used in
   the animation property or one of its longhands, treat the custom
   property as having its initial value for the rest of this algorithm.
2. If the value of the custom property named by the first argument to the
   var() function is anything but the initial value, replace the var()
   function by the value of the corresponding custom property. Otherwise,
3. if the var() function has a fallback value as its second argument,
   replace the var() function by the fallback value. If there are any
   var() references in the fallback, substitute them as well.
4. Otherwise, the property containing the var() function is invalid at
   computed-value time

Our code will then look like this:

:root {  --color:rgba(25,25,25,0.5); /*defined as the default value*/}
.box {  width:50px;  height:50px;  display:inline-block;  margin-right:30px;  border-radius:50%;  position:relative;}.red {background:rgba(255,0,0,0.5);}.blue {background:rgba(0,0,255,0.5);}
.box:before{  content:"";  position:absolute;  top:0;left:0;right:0;bottom:0;  border-radius:50%;  transform:translateX(30px);  background:var(--color,inherit);  filter:invert(1);}

<div class="box red" style="--color:initial;"></div><div class="box blue" style="--color:initial;"></div>
<div class="box" style="background:grey;--color:initial;"></div>

customElements.define( 'custom-icon', class extends HTMLElement {  constructor() {    super()    let size = 100    this.attachShadow( { mode: 'open' } )        .innerHTML = `          <style>            :host {               display: inline-block ;               height: ${size}px ;               width: ${size}px ;               background-color: firebrick ;                color: white;            }          </style>          <slot></slot>`  }} )

custom-icon#i1 {  --icon-size: 50px;  height: var(--icon-size);  width: var(--icon-size);}

<custom-icon id="i1">sized</custom-icon><hr><custom-icon>default</custom-icon>

.box {  --c:inherit;  color:var(--c,red);}

<div>  <div class="box">I am a text</div></div><div style="--c:blue">  <div class="box">I am a text</div></div>