Reminder: MathML meeting today

Apologies for the last minute reminder.

The meeting details were sent to the W3C members-only "member-math" mailing
list <https://lists.w3.org/Archives/Member/member-math/2021May/0000.html> for
the group. The regulars for this group should have the meeting details in
their calendars.

Agenda
1. Announcements/Updates/Progress reports
2. Some progress on ":structure" -- feedback?
3. What properties (and maybe intent names) should be in core?