css not valid

hello

it seems your css not valid, it can’t pass css validator
in xoo-el-style.css

/**
 * Password strength meter
 */
.woocommerce-password-strength {
    text-align: center;
    font-weight: 600;
    padding: 3px 0.5em;
    font-size: 1em;

    &.strong {
        background-color: #c1e1b9;
        border-color: #83c373;
    }

    &.short {
        background-color: #f1adad;
        border-color: #e35b5b;
    }

    &.bad {
        background-color: #fbc5a9;
        border-color: #f78b53;
    }

    &.good {
        background-color: #ffe399;
        border-color: #ffc733;
    }
}

& is less/sacss

Regards