What are the markup languages ​​without strict nesting?

Question

Database

What are the markup languages, like XML, only without strict nesting?
Let me give you an example of a calendar.
If it is compiled in XML markup language, then the nesting will be strict, year> month> (week + day) or year> week> (month + day), but it will not work to merge, so that you can add child elements to both weeks and months days, because one week and its nested elements days, at the junction of months can belong to different months.
I will illustrate with a picture:

Therefore, I am looking for a tool in which this is possible. the main requirement is the internal representation of the data in text format, as in XML.

Answer 1

Sgml, according to this:
https://en.m.wikipedia.org/wiki/Overlapping_markup

Answer 2

JSON

Answer 3

What does "without strict nesting" mean? Nesting is either there or it isn't.
It's easy to make your structure in XML, describing years, months and weeks separately, indicating how many days each element takes:

<calendar>
  <years>
    <year days="4">
    <year days="6">
    <year days="1">
  </years>
  <months>
    <month days="2">
    <month days="4">
    <month days="5">
  </months>
  <weeks>
    <week days="1">
    <week days="2">
    <week days="2">
    <week days="2">
    <week days="2">
    <week days="2">
  </weeks>
</calendar>

Answer 4

well, let's say a week can be nested in two months - the first part in the first month, the second in the second.
when displaying, combine weeks with an identical number or some parameter indicating the separation between months.
variant 2: in general to refuse sections on months and weeks in a year. those. inside the year there is a set of separate days that have two parameters - which month and week they are included in (although this can also be calculated from the number of the day in the year)
and group and show beautifully when displayed.

Answer 5

Judging by the Wishlist, you do not need a markup language, but simply a database and a muzzle to it. In this case, take ms access or libreoffice base, in it, using handles for any article, you will make the database and the interface convenient for you with all the Wishlist like grouping / selection as you like.