Currently this change is barely beneficial (if at all), but it doesn't
harm and might be useful in some cases. For instance, the `datetime`
attributes generates a date containing time zone information, which the
text shown on the screen does not, and styling dates with CSS is now
easier.
This makes the table easier to identify when writing tests and using
screen readers.
Since we do not render any other table captions anywhere else, we're
making the caption invisible so only screen reader users will be
affected by this change.
