Case‑Sensitive or Case‑Insensitive Markdown Link References: Choosing Spec Compliance vs. Legacy Compliance

Determine whether Markdown parsers should enforce the CommonMark‑specified case‑insensitive matching for link reference definitions or provide a configurable case‑sensitive mode to accommodate legacy content that depends on exact case.

The goal is to balance spec compliance, which improves cross‑engine consistency and authoring ergonomics, against the risk of breaking existing documents that were written for case‑sensitive engines and the added complexity of supporting a legacy mode.

Should the default behavior follow the CommonMark spec (case‑insensitive) with an opt‑in flag for case‑sensitive matching, or should parsers default to case‑sensitive and offer a spec‑compliant switch? How should documentation and versioning guide authors when migrating between parsers that differ in reference‑case handling?