[html] How do I simulate placeholder functionality on input date field?

You can use CSS's before pseudo.

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.dateclass {
  width: 100%;
}

.dateclass.placeholderclass::before {
  width: 100%;
  content: attr(placeholder);
}

.dateclass.placeholderclass:hover::before {
  width: 0%;
  content: "";
}
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<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>

<input
  type="date"
  placeholder="Please specify a date"
  onClick="$(this).removeClass('placeholderclass')"
  class="dateclass placeholderclass">
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FIDDLE