A themed <input type="date">. CSS themes the field,
border, focus ring, padding, font, accent. The popup calendar is the
browser's own UI (non-themeable, varies by platform). The value submits
with the surrounding form, no JavaScript.
<div class="casca-date-field">
<label class="casca-date-label" for="start">Start date</label>
<input class="casca-date" type="date" id="start" name="start" value="2026-05-29">
</div>