The page describing the use of input type=‘date’ describes
readonly as if it was a special attribute relevant to
type=‘date’, like the min and max
attributes, rather than as a global attribute applicable to all
form elements.
Also this description includes a paragraph which is repeated
elsewhere on the site:
Note: Because a read-only field cannot have a
value, required does not have any effect on inputs
with the readonly attribute also specified.
But of course read-only fields can and almost
always do have a value, but that value cannot be modified by the
user. This paragraph should read “The required
attribute is not permitted on inputs with the readonly
attribute specified.” The browser should log a diagnostic warning
thatrequired, and all of the other constraint
attributes, is ignored for input elements which have
readonly because the HTML5 standard says “if the
readonly attribute is specified on an input element, the
element is barred from constraint validation.”.