css 浮动 select 单击时的字段标题

Question

我想让 select 字段的行为与其他两个输入字段一样。用户填写字段后，字段标题浮动在顶部。如何使 select 项的行为也像其他两个输入字段一样？这样通过单击该字段，占位符应浮动到顶部。作为占位符显示的默认值是 您的来信，它将在 select 输入一封信或单击该字段后向上移动。

body {
  font-family: Avenir Next, Avenir, SegoeUI, sans-serif;
}


form {
  margin: 2em 0;
}
.field {
  display: flex;
  flex-flow: column-reverse;
  margin-bottom: 1em;
}
label, input {
  transition: all 0.2s;
  touch-action: manipulation;
}
input {
  font-size: 1.5em;
  border: 0;
  border-bottom: 1px solid #ccc;
  font-family: inherit;
  -webkit-appearance: none;
  border-radius: 0;
  padding: 0;
  cursor: text;
}

input:focus {
  outline: 0;
  border-bottom: 1px solid #666;
}

label {
  text-transform: uppercase;
  letter-spacing: 0.05em;
}
input:placeholder-shown + label {
  cursor: text;
  max-width: 66.66%;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
  transform-origin: left bottom;
  transform: translate(0, 2.125rem) scale(1.5);
}
::-webkit-input-placeholder {
  opacity: 0;
  transition: inherit;
}
input:focus::-webkit-input-placeholder {
  opacity: 1;
}
input:not(:placeholder-shown) + label,
input:focus + label {
  transform: translate(0, 0) scale(1);
  cursor: pointer;
}

 form p select, form p select.selectField {
 width: 195px;
 padding: 1px 3px;
}

  <form action="">
  <div class="field">
    <input type="text" name="fullname" id="fullname" placeholder="Jane Appleseed">
    <label for="fullname">Name</label>
  </div>
  
  <div class="field">
    <input type="email" name="email" id="email" placeholder="jane.appleseed@example.com">
    <label for="email">Email</label>
  </div>
    <div id="lettersSelection" >
      <p class="required">
        <select name="letters" id="letters" class="selectField" required="">
          <option value="" selected disabled hidden>Your Letter</option>
          <option value="A">Letter A</option>
          <option value="B">Letter B</option>
          <option value="C">Letter C</option>
        </select>
        <label for="letters">Letters</label>
      </p>
    </div>
  </form>

Answer 1

这正是您所要求的。

我已经在 CSS 中标记了进行更改的行。
我还在 HTML:

中添加了一个额外的元素

<span class="placeholder">Your Letter</span>

body {
  font-family: Avenir Next, Avenir, SegoeUI, sans-serif;
}

form {
  margin: 2em 0;
}

.field {
  display: flex;
  flex-flow: column-reverse;
  margin-bottom: 1em;
}

label{
  display: inline-block; /* <--- needed to have display other than default "inline" */
}

label,
input,
.placeholder{ /* <--- added ".placeholder" */
  transition: .2s;
  touch-action: manipulation;
}

input {
  font-size: 1.5em;
  border: 0;
  border-bottom: 1px solid #ccc;
  font-family: inherit;
  -webkit-appearance: none;
  border-radius: 0;
  padding: 0;
  cursor: text;
}

input:focus {
  outline: 0;
  border-bottom: 1px solid #666;
}

label {
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

input:placeholder-shown+label, 
.placeholder { /* <--- added ".placeholder" */
  cursor: text;
  max-width: 66.66%;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
  transform-origin: left bottom;
  transform: translate(0, 2.125rem) scale(1.5);
}

::-webkit-input-placeholder {
  opacity: 0;
  transition: inherit;
}

input:focus::-webkit-input-placeholder {
  opacity: 1;
}

input:not(:placeholder-shown)+label,
input:focus+label {
  transform: none; /* <--- changed to "none" to restore defaults */
  cursor: pointer;
}

form p select,
form p select.selectField {
  width: 195px;
  padding: 5px;  /* <--- bigger value to cover the placeholder text */
}

/* ------- ADDED ------ */
#lettersSelection{ position:relative; margin-top:3em; }

#lettersSelection .placeholder{
  position: absolute;
  top: -1.8rem;
  left: 0;
}

select{ position:relative; z-index:1; }

select:focus + .placeholder{
  transform: none; /* <--- moves the title up on "focus" */
}

<form action="">
  <div class="field">
    <input type="text" name="fullname" id="fullname" placeholder="Jane Appleseed">
    <label for="fullname">Name</label>
  </div>

  <div class="field">
    <input type="email" name="email" id="email" placeholder="jane.appleseed@example.com">
    <label for="email">Email</label>
  </div>
  
  <div id="lettersSelection">
    <p class="required">
      <select name="letters" id="letters" class="selectField" required>
        <option value="" selected disabled hidden>Your Letter</option>
        <option value="A">Letter A</option>
        <option value="B">Letter B</option>
        <option value="C">Letter C</option>
      </select>
      <span class="placeholder">Your Letter</span>
      <label for="letters">Letters</label>
    </p>
  </div>
</form>

css 浮动 select 单击时的字段标题

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这正是您所要求的。