如何使 HTML 选择框选项在悬停时可见？

<code class="js">$('#selectUl li:not(":first")').addClass('unselected'); // Hide unselected elements

$('#selectUl').hover(
  function() { // Mouseover event
    $(this).find('li').click(
      function() {
        $('.unselected').removeClass('unselected'); // Remove unselected styles

        $(this).siblings('li').addClass('unselected'); // Add unselected styles to other elements

        var index = $(this).index(); // Get the index of the clicked option
        $('select option:selected').removeAttr('selected'); // Deselect the previously chosen option

        $('select[name=size]')
          .find('option:eq(' + index + ')')
          .attr('selected', true); // Select the new option
      }
    );
  },
  function() { // Mouseout event
    // Hide unselected elements
  }
);</code>

<code class="css">select {
  opacity: 0.5;
}
ul {
  width: 8em;
  line-height: 2em;
}

li {
  display: list-item;
  width: 100%;
  height: 2em;
  border: 1px solid #ccc;
  border-top-width: 0;
  text-indent: 1em;
  background-color: #f90;
}
li:first-child {
  border-top-width: 1px;
}

li.unselected {
  display: none;
  background-color: #fff;
}
ul#selectUl:hover li.unselected {
  background-color: #fff;
}
ul#selectUl:hover li,
ul#selectUl:hover li.unselected {
  display: list-item;
}
ul#selectUl:hover li {
  background-color: #fc0;
}
ul#selectUl li:hover,
ul#selectUl li.unselected:hover {
  background-color: #f90;
}</code>

<code class="js">(function($) {
  $.fn.selectUl = function() {
    // ... code goes here ...
    return $(this);
  };
})(jQuery);

$('#sizes').selectUl();</code>