Placeholder text is a hint, not a replacement for a visible label.
<form>
<label for="first-name">First name</label>
<input id="first-name" type="text" name="first_name" placeholder="e.g. Alex">
<label for="last-name">Last name</label>
<input id="last-name" type="text" name="last_name" placeholder="e.g. Taylor">
</form>