Add <template contentmethod> for declarative out-of-order streaming

[foolip]

Fixes #11542.

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[noamr]

It's a bit more involved because we need to adjust the location based on the grandparent's contentmethod and the parent's contentname.

[noamr]

That's the "adjusted insertion location" or whatever it's called

[foolip]

I've made some additional changes but things don't quite make sense yet. The direction I'm heading in is:

• When the parser encounters an <template contentmethod> element, it doesn't insert it.
• In insert an element at the adjusted insertion location, if we're about to insert an element with a contentname attribute into such a <template> element:
  • Find the target element among the descendents of the element that the <template> element was in. That target is kept as a bookkeeping slot, the tree traversal only happens once.
  • For contentmethod=replace, remove the target and actually insert. (For the following cases, the element isn't inserted.)
  • For contentmethod=replace-children, remove the children.
  • For contentmethod=prepend, save the current first element of the target.
  • For contentmethod=append, there's nothing to do here.
• In appropriate place for inserting a node further adjust the location based on contentmethod after the existing foster parenting adjustments. (This happens before the above step, but isn't relevant in that case.)
• In insert an element at the adjusted insertion location, if we're about to insert into an element with a contentname attribute and the bookkeeping all checks out:
  • For contentmethod=replace-children and contentmethod=append, just append.
  • For contentmethod=prepend, use the saved first element of the target.

There are options for where to store the bookkeeping. I initially put it on the <template> element but now think it might be easier to follow if the bookkeeping goes on the element with the contentname attribute. An implementation might keep it as extra information in the stack of open elements.

[noamr]

I've made some additional changes but things don't quite make sense yet. The direction I'm heading in is:

• When the parser encounters an <template contentmethod> element, it doesn't insert it.

contentmethod also needs to be valid

• In insert an element at the adjusted insertion location, if we're about to insert an element with a contentname attribute into such a <template> element:

  • Find the target element among the descendents of the element that the <template> element was in. That target is kept as a bookkeeping slot, the tree traversal only happens once.
  • For contentmethod=replace, remove the target and actually insert. (For the following cases, the element isn't inserted.)
  • For contentmethod=replace-children, remove the children.
  • For contentmethod=prepend, save the current first element of the target.
  • For contentmethod=append, there's nothing to do here.

• In appropriate place for inserting a node further adjust the location based on contentmethod after the existing foster parenting adjustments. (This happens before the above step, but isn't relevant in that case.)

• In insert an element at the adjusted insertion location, if we're about to insert into an element with a contentname attribute and the bookkeeping all checks out:

  • For contentmethod=replace-children and contentmethod=append, just append.
  • For contentmethod=prepend, use the saved first element of the target.

It also needs to fail if that first element is no longer a child of the target.

There are options for where to store the bookkeeping. I initially put it on the <template> element but now think it might be easier to follow if the bookkeeping goes on the element with the contentname attribute. An implementation might keep it as extra information in the stack of open elements.

Yea makes sense, that way you don't have to deal with grandparents.