Fix issue of calc for .form-required::after which not displayed with error

Compiled CSS:

    -webkit-background-size: calc(1.5em + 0.75rem + 2px)/2 calc(1.5em + 0.75rem + 2px)/2;
    background-size: calc(1.5em + 0.75rem + 2px)/2 calc(1.5em + 0.75rem + 2px)/2;
    width: calc(1.5em + 0.75rem + 2px)/2;
    height: calc(1.5em + 0.75rem + 2px)/2;

Error: Invalid property value.

May be:

    -webkit-background-size: calc((1.5em + 0.75rem + 2px)/2) calc((1.5em + 0.75rem + 2px)/2);
    background-size: calc((1.5em + 0.75rem + 2px)/2) calc((1.5em + 0.75rem + 2px)/2);
    width: calc((1.5em + 0.75rem + 2px)/2);
    height: calc((1.5em + 0.75rem + 2px)/2);

calc in ()