<form>
  <div class="form-container">
    <div class="form-item">
      <label for="form-firstname">First name</label>
      <input type="text" id="form-firstname">
    </div>
    <div class="form-item">
      <label for="form-lastname">Last name</label>
      <input type="text" id="form-lastname">
    </div>
    <div class="form-item">
      <label for="form-email">Email address</label>
      <input type="email" id="form-email" aria-describedby="form-email-helper">
      <p class="form-helper" id="form-email-helper">Expected format: ada.lovelace@codelikealady.com</p>
    </div>
  </div>
</form>

body {
  font-family: Arial, sans-serif;
}

* {
  box-sizing: border-box;
}

form {
  background: #faf8f4;
  max-width: 800px;
  padding: 2rem;
}

.form-container {
  container: form / inline-size;
}

.form-item + .form-item {
  margin-top: 1.5rem;
}

label {
  display: block;
  font-size: 1.25rem;
  margin-bottom: 0.25em;
}

input[type="text"],
input[type="email"] {
  background: #fff;
  border: 1px solid #564f6c;
  border-radius: 2px;
  font-size: 1.25rem;
  padding: 0.25em 0.5em;
  width: 100%;
}

input:focus {
  outline: 2px solid #594a87;
  outline-offset: 1px;
}

.form-helper {
  font-size: 0.9375rem;
  margin: 0.5em 0 0;
}

@container form (min-width: 600px) {
  .form-item {
    display: flex;
    align-items: center;
    flex-wrap: wrap;
  }
  label {
    flex-basis: 25%;
    margin: 0;
    text-align: right;
    padding-right: 0.75em;
    width: 25%;
    vertical: middle;
  }
  input[type="text"],
  input[type="email"] {
    flex-grow: 1;
    width: auto;
  }
  .form-helper {
    flex-basis: 100%;
    padding-left: 25%;
  }
}