So... you'll have to write Javascript that goes and replaces the additional markers with the stuff they're supposed to expand to? Because what I see there is just a state of partial evaluation, and that leads me to ask (and would lead anyone to ask) how evaluation of all substitutions can be completed...
At the moment - you can treat it as a single "search & replace", and not as a replace all. And yes JS can do the rest atm.
Hopefully introducing this (and processing instructions in general) can lead to additional enhancements in the future. But it would have to be gradual, kind of like how modern CSS evolved.
It just feels half-done. This seems to be a loose end left hanging because this is all rooted in theory and not in practice. In theory it doesn't matter if you have to polyfill half the builtin feature with JS because otherwise it doesn't work. In practice, yes it matters.
<template for="placeholder2"> <?marker name="placeholder1"> <?marker name="placeholder1"> </template>
<template for="placeholder1"> <?marker name="placeholder"> <?marker name="placeholder"> </template>
<template for="placeholder"> Here is some <em>HTML content</em>! </template>
Before you couldn't make this dangerous (without JS) because there was no way to auto expand the templates