How can we prevent a specific element from inheriting CSS rules with * sign?

Question

I have defined this CSS rule:

*,
*::before,
*::after {
    box-sizing: border-box
}

What if we want to exclude only a specific element with this class .testim?

Answer 1

Its very simple; If I correct undertood then;

.testim,
.testim::before,
.testim::after{
  box-sizing: unset;
}

The unset CSS keyword resets a property to its inherited value if the property naturally inherits from its parent, and to its initial value if not.

More info for CSS unset value.

Answer 2

:not() is like *. It selects every element apart from the element in its brackets. (https://developer.mozilla.org/en-US/docs/Web/CSS/:not).

:not(.testim), *::before, *::after {
  box-sizing: border-box;
}

div {
  width: 200px;
  border: 20px solid black;
}

.testim {
  width: 200px;
  border: 20px solid black;
}

<div>I am div with width: 200px and border: 20px.</div>
<hr>
<div class="testim">I am of the class testim with width: 200px and border: 20px.</div>