org-html-preamble-format

org-html-preamble-format is a variable defined in `ox-html.el'.
Its value is

(("en" ""))

Documentation:
Alist of languages and format strings for the HTML preamble.

The first element of each list is the language code, as used for
the LANGUAGE keyword. See `org-export-default-language'.

The second element of each list is a format string to format the
preamble itself. This format string can contain these elements:

%t stands for the title.
%a stands for the author's name.
%e stands for the author's email.
%d stands for the date.
%c will be replaced by `org-html-creator-string'.
%v will be replaced by `org-html-validation-link'.
%T will be replaced by the export time.
%C will be replaced by the last modification time.

If you need to use a "%" character, you need to escape it
like that: "%%".

See the default value of `org-html-postamble-format' for an
example.

You can customize this variable.