Lets drop this one
I know this is probably going to be an unpopular opinion, but the more I think about this issue, the more it strikes me as something wholly external to markdown.
Would it be convenient if markdown automatically inserted implicit IDs into headers? sure, but it runs the risk of conflicting with whatever external environment the generated markup is being added into.
Instead of trying to incorporate this feature into markdown where it might cause more bugs and make authoring more complicated, let’s just drop this feature request.
Of course, I’m not saying we shouldn’t autogenerate [id] attributes. On the contrary, autogenerate IDs as a post-process step.
That way it can be customized to suit the needs of the environment. That way we don’t have to try to add options to all markdown processors that are only meant for ID resolution.
Implementing a post-processor for this is trivially easy. Throw the rendered markup into a DOM parser, grab all the h* elements, and add ID attributes based on whatever ID resolution scheme suits your needs.
# Lorem ipsum
## Dolor sit amet
could produce
<h1 id="lorem-ipsum">Lorem ipsum</h1>
<h2 id="dolor-sit-amet">Dolor sit amet</h2>
if your resolution scheme is content based (which I assume will be the most popular)
or it could produce
<h1 id="1">Lorem ipsum</h1>
<h2 id="1.1">Dolor sit amet</h2>
if your resolution scheme is based on hierarchy.
or it could produce…whatever you want because it’s completely customizable.
In either case, the post-process step can easily be customized and easily debugged because it’s a smaller, separate entity to markdown.