Heading identifiers in Pandoc-style Markdown

Pandoc introduced the syntax:

# Heading title {#custom-id}

to allow authors to define explicit heading identifiers.

What supports it

Supported in:

• Pandoc (the defining implementation).
• Hugo (via Goldmark with the attribute extension enabled).
• MkDocs / Material for MkDocs (through the Python Markdown attr_list extension).
• Obsidian (supports reading these IDs though not all plugins write them).

What does not support it (natively)

• GitHub Flavored Markdown rendering on github.com (it strips the {#id} block).
• CommonMark and the CommonMark spec do not include this extension.
• Many simpler Markdown renderers (including some JavaScript ones) ignore it.

Pandoc allows attribute blocks only in specific syntactic positions, not anywhere.

The authoritative specification is in the Pandoc User’s Guide (“Extension: attr_parser” and the descriptions of header_attributes, fenced_code_attributes, etc.).

Where Pandoc allows attributes

1. Headings

   ## Title {#id .class key=val}
   

2. Fenced code blocks

   ```lang {#id .class}
   

3. Images (using the general attribute syntax after the link)

   ![alt](img.png){#id .class}
   

4. Div / span elements via fenced syntax

   ::: {.myclass}
   Content
   :::
   

5. Inline spans

   [text]{.class}
   

Where Pandoc does not allow attributes

• You cannot attach arbitrary {#id} immediately after ordinary paragraphs, list items, blockquotes, or arbitrary inline text unless you are using the span or div syntaxes above.
• You cannot place {#id} in the middle of a paragraph and expect it to attach to the paragraph itself.

Minimal conclusion

Pandoc’s {#id} syntax is not universal; it works only in constructs that explicitly support attributes (headings, code blocks, images, spans, and divs).