@import url('https://fonts.google.com/specimen/Lato?selection.family=Lato');
 :root {
  --main-color: #ee6e73;
  --second-color: #333;
  --input-color: #9e9e9e;
}

html,
body,
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-size: 16px;
}

body {
  height: 100vh;
  /* assumed you want to super center the form */
  display: grid;
  /* same as center of both align/justify ref
https://developer.mozilla.org/en-US/docs/Web/CSS/place-items */
  place-items: center;
  font-family: 'Lato', sans-serif;
  background-color: #fcfcfc;
}

.form-form {
  padding-top: 1.25em;
  padding-bottom: 1.25em;
  padding-left: 1.5em;
  padding-right: 1.5em;
  display: grid;
  grid-template-columns: 1fr;
  /* fieldset takes whatever space is left over - 1fr */
  grid-template-rows: auto 1fr 1em auto;
  grid-template-areas: "formheader" "forminputs" "." "formbutton";
  align-items: center;
  border: solid cyan 1px;
  width: 100%;
}

.form-form .form-header {
  grid-area: formheader;
  color: var(--second-color);
  letter-spacing: 1px;
  margin-top: 0.625em;
  margin-bottom: 0.625em;
}

.fieldset-container {
  grid-area: forminputs;
  display: grid;
  grid-template-rows: 1fr;
  /* width of each field/column */
  grid-template-columns: repeat(3, 18.75em);
  border-bottom: 1px solid var(--input-color, #00FF00);
  padding-bottom: 0.5em;
  margin-bottom: 0.5em;
}

.form-group {
  padding-top: 1.25em;
  /*padding-bottom: 1.25em;*/
  display: grid;
  grid-template-columns: [only-one] 1fr;
  grid-template-rows: [form-things] auto 1fr;
  /* we COULD use the areas but we want to put them in the same line */
  grid-template-areas: "labels" "inputs";
}

.form-group .form-group-label {
  /* could also use the named line/row */
  grid-area: labels;
  padding-top: 1.25em;
  padding-bottom: 1.25em;
  color: var(--input-color);
  font-weight: 100;
  pointer-events: none;
  background-color: transparent;
}

.form-group .form-group-input {
  /* we named the line/column so use that not the "area" name */
  /*  grid-area: inputs;*/
  grid-row-start: form-things;
  grid-column-start: only-one;
  background-color: transparent;
  border: none;
  color: var(--second-color);
  padding-top: 1.25em;
  padding-bottom: 1.25em;
}

.form-group .form-group-input:focus,
{
  border-bottom-color: var(--main-color);
  outline: none;
}

.form-group .form-group-input:valid~label,
.form-group .form-group-input:focus~label {
  color: var(--main-color);
  font-size: 0.875em;
  transform: translateY(0);
}

.form-button {
  grid-area: formbutton;
  background-color: var(--main-color);
  border: 2px solid var(--main-color);
  /* made bigger as the 2px did not really show */
  border-radius: 0.5em;
  color: #fff;
  cursor: pointer;
  height: 3em;
  /* moved to grid as a row but not named area "."
 margin-top: 1em;*/
}

.form-button:hover {
  background-color: #fff;
  color: var(--main-color);
}

.form-button:active {
  border-color: #fff;
}

.form-button:focus {
  outline: none;
  border-color: #00FF00;
}

<form class="form-form">
  <h1 class="form-header">CSS Floating Labels</h1>
  <fieldset>
    <div class="fieldset-container">
      <div class="form-group">
        <input id="form-input-1" class="form-group-input" type="text" required/>
        <label for="form-input-1" class="form-group-label">Username</label>
      </div>
      <div class="form-group">
        <!-- <label class="form-group-label">TesterOut</label> -->
        <select class="form-group-input">
          <option value="">Test</option>
          <option value="water">Water</option>
          <option value="milk">Milk</option>
        </select>
      </div>
      <div class="form-group">
        <input class="form-group-input" type="password" required/><label class="form-group-label">Password</label>
      </div>
    </div>
  </fieldset>
  <button class="form-button" type="button">Submit</button>
</form>