diff --git a/src/core/idna/codegen.py b/src/core/idna/codegen.py
index a203874e7..65c64fff3 100644
--- a/src/core/idna/codegen.py
+++ b/src/core/idna/codegen.py
@@ -11,7 +11,7 @@
 PAGE_SIZE = 1 << PAGE_SHIFT
 NUM_PAGES = TOTAL_CODEPOINTS // PAGE_SIZE
 
-# Integer values must match the IdnaProperty enum in idna_ucd.h.
+# Integer values must match the IDNAProperty enum in idna_ucd.h.
 IDNA_PROPERTY_VALUES = {
     "PVALID": 0,
     "CONTEXTJ": 1,
diff --git a/src/core/idna/idna.cc b/src/core/idna/idna.cc
index d9f932966..d0db13af7 100644
--- a/src/core/idna/idna.cc
+++ b/src/core/idna/idna.cc
@@ -149,7 +149,6 @@ auto idna_passes_contextj(const std::u32string_view label,
   return true;
 }
 
-// TODO: Reject labels that are not NFC-normalized
 auto idna_is_valid_u_label(const std::u32string_view label) noexcept -> bool {
   if (label.empty()) {
     return false;
@@ -257,7 +256,9 @@ auto idna_passes_bidi_rule(const std::u32string_view label) noexcept -> bool {
     }
   }
 
-  // RFC 5893 §2 condition 4: an RTL label cannot have both EN and AN.
+  // RFC 5893 §2 condition 4 (RTL labels only): cannot have both EN and AN.
+  // The "and vice versa" wording in the RFC is internal to the RTL branch
+  // (EN/AN exclusivity within RTL), not symmetry to LTR.
   if (is_rtl_label && has_european_number && has_arabic_number) {
     return false;
   }
diff --git a/src/core/idna/include/sourcemeta/core/idna.h b/src/core/idna/include/sourcemeta/core/idna.h
index d6f2c7406..8f5fdf30e 100644
--- a/src/core/idna/include/sourcemeta/core/idna.h
+++ b/src/core/idna/include/sourcemeta/core/idna.h
@@ -41,8 +41,9 @@ auto idna_property(const char32_t codepoint) noexcept -> IDNAProperty;
 /// @ingroup idna
 /// Return whether the codepoint at `position` within `label` does not
 /// violate any RFC 5892 Appendix A.3-A.9 contextual rule. Returns true
-/// vacuously when the codepoint has no such rule. See
-/// https://www.rfc-editor.org/rfc/rfc5892#appendix-A for the rules.
+/// vacuously when the codepoint has no such rule. Returns false when
+/// `position` is out of range, treated as a precondition violation.
+/// See https://www.rfc-editor.org/rfc/rfc5892#appendix-A for the rules.
 /// For example:
 ///
 /// ```cpp
@@ -59,8 +60,9 @@ auto idna_passes_contexto(const std::u32string_view label,
 /// @ingroup idna
 /// Return whether the codepoint at `position` within `label` does not
 /// violate any RFC 5892 Appendix A.1 / A.2 contextual rule. Returns true
-/// vacuously when the codepoint has no such rule. See
-/// https://www.rfc-editor.org/rfc/rfc5892#appendix-A for the rules.
+/// vacuously when the codepoint has no such rule. Returns false when
+/// `position` is out of range, treated as a precondition violation.
+/// See https://www.rfc-editor.org/rfc/rfc5892#appendix-A for the rules.
 /// For example:
 ///
 /// ```cpp
@@ -102,7 +104,9 @@ auto idna_passes_bidi_rule(const std::u32string_view label) noexcept -> bool;
 /// Return whether the given label is a valid U-label per RFC 5891 §4. See
 /// https://www.rfc-editor.org/rfc/rfc5891#section-4 for the criteria.
 /// The Bidi rule is not checked here because Bidi domain detection is a
-/// property of the whole domain, not of a single label. For example:
+/// property of the whole domain, not of a single label. Callers needing
+/// strict conformance must normalise the input to NFC themselves. The
+/// RFC 5891 §4.1.2.A NFC requirement is not enforced. For example:
 ///
 /// ```cpp
 /// #include <sourcemeta/core/idna.h>