::placeholder

input {
  margin-top: 0.5rem;
}

input::placeholder {
  font-weight: bold;
  opacity: 0.5;
  color: red;
}

<label for="first-name">Your phone number:</label><br />

<input
  id="first-name"
  type="tel"
  name="phone"
  minlength="9"
  maxlength="9"
  placeholder="It must be 9 digits" />

::placeholder {
  /* ... */
}

<label for="user-email">Your email address</label>
<span id="user-email-hint" class="input-hint">Example: jane@sample.com</span>
<input
  id="user-email"
  aria-describedby="user-email-hint"
  name="email"
  type="email" />

<input placeholder="Type here" />

input::placeholder {
  color: red;
  font-size: 1.2em;
  font-style: italic;
  opacity: 0.5;
}

<input placeholder="Color set by browser" />
<input placeholder="Same color as input" class="explicit-color" />
<input placeholder="Semi-opaque text color" class="opacity-change" />

input {
  font-weight: bold;
  color: green;
}

.explicit-color::placeholder {
  /* use the same color as input element to avoid the browser set default color */
  color: currentColor;
}

.opacity-change::placeholder {
  /* less opaque text */
  color: color-mix(in srgb, currentColor 70%, transparent);
}