试试这个

<script>
    document.addEventListener("DOMContentLoaded", () => {
        var el = document.querySelector('#quantity');
        el.addEventListener('input', function(){
            let val = this.value;
            let max = parseFloat(this.getAttribute('max'));
            val = parseFloat(val);
            if(isNaN(val)) this.value = '';
            if(val > max) this.value = max;
        });
    });
</script>