ISO 8601 is not related to Unicode (ISO/IEC 10646), but if you do care about Unicode you may notice that Unicode CLDR actually supports the preferred first day of the week as a part of locale identifiers [1] (e.g. en-US-u-fw-mon will make the first day of the week Monday, as opposed to the default Sunday for USA).
The definition of Monday as the first day of the week is strictly for the sake of week numbering over the course of the year and does not indicate that a calendar which displays weeks starting with Sunday violates the ISO standard.
This is a lovely little project but it makes no claims to be for markdown—the site makes no mention of it as far as I can see. And, of course these fun unicode calendars don't render in Markdown.
Can we change the title back to "Unicode Calendar Generator"?
"markdown" is not to be found on the page, so it's not for SEO; the submitter (incorrectly) editorialized the title. It's also not restricted to ASCII, so your suggestion does not increase title accuracy.
https://en.m.wikipedia.org/wiki/ISO_8601