An example of a valid CSS stylesheet using @--reset, --clamped-font-size and nesting CSS rules inside CSS custom properties, can be built with process-css-demo or online at https://tomhodgins.com/demo/preprocess/

test-9.css

@--reset test-9;

.up-modal {
  z-index: 9999999;
}

test-9 {
  display: flex;
  flex-direction: column;
  align-items: center;

  ---\ h1: {
    margin: .5em auto;
    max-width: 100%;
    color: black;
    --clamped-font-size: 24, 6.75, 40;
    font-weight: 500;
    line-height: 1.2;
    letter-spacing: 0;
    text-align: center;

    ---\ strong: {
      font-weight: bolder;
    };
  };

  ---\ a: {
    display: block;
    margin: 1em auto;
    font-family: Montserrat;
    font-size: 24px;
    font-weight: 500;
    border-radius: 1em;
    border: 1px solid #f8b34f;
    display: inline-block;
    padding: 8px 30px;
    color: white;
    text-transform: uppercase;
    background: #f8b34f;
    text-shadow: rgba(0, 0, 0, .1) 0 0 5px;

    ---\:hover\,\:focus: {
      background: #FFFFFF;
      color: #4a4a4a;
      text-decoration: none;
    };

    ---\:active: {
      box-shadow: inset 0 6px 10px rgba(0, 0 , 0, .125);
    };
  };
}