author here, just to note our current design pattern for this is slightly updated:
https://design-system.service.gov.uk/components/date-input/
I was wondering why you chose to use the placeholder attribute to show labels rather than the (afaik recommended) usage of showing example values. The form shows example values, but outside the input fields. Did you test both ways and this turned out to be better after all, or this was just an implementation which worked fine in testing so it stuck?