This is tightly related to Block reference, but much simpler to add as a feature.
Why can’t we link to a subheader located in another note? Let’s say you have a note with “three principles of X”, each principle being its own subheader (## Principle 1 and so worth).
If you’re writing another note and want to reference just one of such principles, I don’t see the point of having to separate the “three principles of X” into three separate notes in order to be able to reference a specific one.
This seems plausible to achieve using Markdown/HTML by writing something like [[three principles of X#Principle One]] (or in HTML: <a href="three-principles-of-x#principle-one"></a>, so in theory it could work across Markdown editors.
It could also be used as a stop-gap until a consensus on block references is established, allowing users that want to use them to add some less important header (like ###### header I want to reference) and being able to link to it from another note, though it may reduce the readability of notes for those that would be heavy users of this feature.