diff --git a/Lib/fontTools/mtiLib/__init__.py b/Lib/fontTools/mtiLib/__init__.py
index 97b91ea1c..13b308e51 100644
--- a/Lib/fontTools/mtiLib/__init__.py
+++ b/Lib/fontTools/mtiLib/__init__.py
@@ -12,6 +12,8 @@ from fontTools.ttLib.tables import otTables as ot
 from fontTools.ttLib.tables.otBase import ValueRecord, valueRecordFormatDict
 from fontTools.otlLib import builder as otl
 from contextlib import contextmanager
+from fontTools.ttLib import newTable
+from fontTools.feaLib.lookupDebugInfo import LOOKUP_DEBUG_INFO_KEY
 from operator import setitem
 import logging
 
@@ -1036,7 +1038,17 @@ def parseGSUBGPOS(lines, font, tableTag):
         self.LookupList.LookupCount = len(self.LookupList.Lookup)
     if lookupMap is not None:
         lookupMap.applyDeferredMappings()
-    if featureMap is not None:
+        if "Debg" not in font:
+            font["Debg"] = newTable("Debg")
+            font["Debg"].data = {}
+        debug = (
+            font["Debg"]
+            .data.setdefault(LOOKUP_DEBUG_INFO_KEY, {})
+            .setdefault(tableTag, {})
+        )
+        for name, lookup in lookupMap.items():
+            debug[str(lookup)] = ["", name, ""]
+
         featureMap.applyDeferredMappings()
     container.table = self
     return container
diff --git a/Tests/mtiLib/data/featurename-backward.ttx.GSUB b/Tests/mtiLib/data/featurename-backward.ttx.GSUB
index cc893cd90..0fbe51fd5 100644
--- a/Tests/mtiLib/data/featurename-backward.ttx.GSUB
+++ b/Tests/mtiLib/data/featurename-backward.ttx.GSUB
@@ -47,6 +47,7 @@
   </FeatureList>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- l1:  -->
     <Lookup index="0">
       <LookupType value="1"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/featurename-forward.ttx.GSUB b/Tests/mtiLib/data/featurename-forward.ttx.GSUB
index cc893cd90..0fbe51fd5 100644
--- a/Tests/mtiLib/data/featurename-forward.ttx.GSUB
+++ b/Tests/mtiLib/data/featurename-forward.ttx.GSUB
@@ -47,6 +47,7 @@
   </FeatureList>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- l1:  -->
     <Lookup index="0">
       <LookupType value="1"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/lookupnames-backward.ttx.GSUB b/Tests/mtiLib/data/lookupnames-backward.ttx.GSUB
index cb358d7c8..811f79a05 100644
--- a/Tests/mtiLib/data/lookupnames-backward.ttx.GSUB
+++ b/Tests/mtiLib/data/lookupnames-backward.ttx.GSUB
@@ -35,6 +35,7 @@
   </FeatureList>
   <LookupList>
     <!-- LookupCount=2 -->
+    <!-- l1:  -->
     <Lookup index="0">
       <LookupType value="1"/>
       <LookupFlag value="0"/>
@@ -44,6 +45,7 @@
         <Substitution in="uvowelsignkannada" out="uvowelsignaltkannada"/>
       </SingleSubst>
     </Lookup>
+    <!-- l0:  -->
     <Lookup index="1">
       <LookupType value="6"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/lookupnames-forward.ttx.GSUB b/Tests/mtiLib/data/lookupnames-forward.ttx.GSUB
index 249d605b6..86b3148f0 100644
--- a/Tests/mtiLib/data/lookupnames-forward.ttx.GSUB
+++ b/Tests/mtiLib/data/lookupnames-forward.ttx.GSUB
@@ -35,6 +35,7 @@
   </FeatureList>
   <LookupList>
     <!-- LookupCount=2 -->
+    <!-- l0:  -->
     <Lookup index="0">
       <LookupType value="6"/>
       <LookupFlag value="0"/>
@@ -74,6 +75,7 @@
         </ChainSubClassSet>
       </ChainContextSubst>
     </Lookup>
+    <!-- l1:  -->
     <Lookup index="1">
       <LookupType value="1"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mixed-toplevels.ttx.GSUB b/Tests/mtiLib/data/mixed-toplevels.ttx.GSUB
index 249d605b6..74192a51d 100644
--- a/Tests/mtiLib/data/mixed-toplevels.ttx.GSUB
+++ b/Tests/mtiLib/data/mixed-toplevels.ttx.GSUB
@@ -35,6 +35,7 @@
   </FeatureList>
   <LookupList>
     <!-- LookupCount=2 -->
+    <!-- 0:  -->
     <Lookup index="0">
       <LookupType value="6"/>
       <LookupFlag value="0"/>
@@ -74,6 +75,7 @@
         </ChainSubClassSet>
       </ChainContextSubst>
     </Lookup>
+    <!-- 1:  -->
     <Lookup index="1">
       <LookupType value="1"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/chained-glyph.ttx.GPOS b/Tests/mtiLib/data/mti/chained-glyph.ttx.GPOS
index b550c700f..32fffc4b7 100644
--- a/Tests/mtiLib/data/mti/chained-glyph.ttx.GPOS
+++ b/Tests/mtiLib/data/mti/chained-glyph.ttx.GPOS
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=2 -->
+    <!-- raucontext-sinh:  -->
     <Lookup index="0">
       <LookupType value="8"/>
       <LookupFlag value="512"/><!-- markAttachmentType[2] -->
@@ -43,6 +44,7 @@
         </ChainPosRuleSet>
       </ChainContextPos>
     </Lookup>
+    <!-- u2aelow-sinh:  -->
     <Lookup index="1" empty="1"/>
   </LookupList>
 </GPOS>
diff --git a/Tests/mtiLib/data/mti/chained-glyph.ttx.GSUB b/Tests/mtiLib/data/mti/chained-glyph.ttx.GSUB
index 7dfdb8480..30a15304d 100644
--- a/Tests/mtiLib/data/mti/chained-glyph.ttx.GSUB
+++ b/Tests/mtiLib/data/mti/chained-glyph.ttx.GSUB
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=2 -->
+    <!-- raucontext-sinh:  -->
     <Lookup index="0">
       <LookupType value="6"/>
       <LookupFlag value="512"/><!-- markAttachmentType[2] -->
@@ -43,6 +44,7 @@
         </ChainSubRuleSet>
       </ChainContextSubst>
     </Lookup>
+    <!-- u2aelow-sinh:  -->
     <Lookup index="1" empty="1"/>
   </LookupList>
 </GSUB>
diff --git a/Tests/mtiLib/data/mti/chainedclass.ttx.GSUB b/Tests/mtiLib/data/mti/chainedclass.ttx.GSUB
index fcd7569f4..39691c448 100644
--- a/Tests/mtiLib/data/mti/chainedclass.ttx.GSUB
+++ b/Tests/mtiLib/data/mti/chainedclass.ttx.GSUB
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=2 -->
+    <!-- swashes-knda:  -->
     <Lookup index="0">
       <LookupType value="6"/>
       <LookupFlag value="0"/>
@@ -42,6 +43,7 @@
         </ChainSubClassSet>
       </ChainContextSubst>
     </Lookup>
+    <!-- u-swash-knda:  -->
     <Lookup index="1">
       <LookupType value="1"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/chainedcoverage.ttx.GSUB b/Tests/mtiLib/data/mti/chainedcoverage.ttx.GSUB
index 4f312c6e2..bea53f50f 100644
--- a/Tests/mtiLib/data/mti/chainedcoverage.ttx.GSUB
+++ b/Tests/mtiLib/data/mti/chainedcoverage.ttx.GSUB
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=2 -->
+    <!-- slashcontext:  -->
     <Lookup index="0">
       <LookupType value="6"/>
       <LookupFlag value="0"/>
@@ -45,6 +46,7 @@
         </SubstLookupRecord>
       </ChainContextSubst>
     </Lookup>
+    <!-- slashTofraction:  -->
     <Lookup index="1">
       <LookupType value="1"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gposcursive.ttx.GPOS b/Tests/mtiLib/data/mti/gposcursive.ttx.GPOS
index 6c08c50c3..6d92d083f 100644
--- a/Tests/mtiLib/data/mti/gposcursive.ttx.GPOS
+++ b/Tests/mtiLib/data/mti/gposcursive.ttx.GPOS
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- kernpairs:  -->
     <Lookup index="0">
       <LookupType value="3"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gposkernset.ttx.GPOS b/Tests/mtiLib/data/mti/gposkernset.ttx.GPOS
index a8371233b..e7a5ff782 100644
--- a/Tests/mtiLib/data/mti/gposkernset.ttx.GPOS
+++ b/Tests/mtiLib/data/mti/gposkernset.ttx.GPOS
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- 0:  -->
     <Lookup index="0">
       <LookupType value="2"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gposmarktobase.ttx.GPOS b/Tests/mtiLib/data/mti/gposmarktobase.ttx.GPOS
index e6e210287..b78d4ff02 100644
--- a/Tests/mtiLib/data/mti/gposmarktobase.ttx.GPOS
+++ b/Tests/mtiLib/data/mti/gposmarktobase.ttx.GPOS
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- topmarktobase-guru:  -->
     <Lookup index="0">
       <LookupType value="4"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gpospairclass.ttx.GPOS b/Tests/mtiLib/data/mti/gpospairclass.ttx.GPOS
index 32b35aee9..9058eb0fd 100644
--- a/Tests/mtiLib/data/mti/gpospairclass.ttx.GPOS
+++ b/Tests/mtiLib/data/mti/gpospairclass.ttx.GPOS
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- 0:  -->
     <Lookup index="0">
       <LookupType value="2"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gpospairglyph.ttx.GPOS b/Tests/mtiLib/data/mti/gpospairglyph.ttx.GPOS
index f03a90e33..58567a971 100644
--- a/Tests/mtiLib/data/mti/gpospairglyph.ttx.GPOS
+++ b/Tests/mtiLib/data/mti/gpospairglyph.ttx.GPOS
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- 0:  -->
     <Lookup index="0">
       <LookupType value="2"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gpossingle.ttx.GPOS b/Tests/mtiLib/data/mti/gpossingle.ttx.GPOS
index c3bdbf68a..3a955f659 100644
--- a/Tests/mtiLib/data/mti/gpossingle.ttx.GPOS
+++ b/Tests/mtiLib/data/mti/gpossingle.ttx.GPOS
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- supsToInferiors:  -->
     <Lookup index="0">
       <LookupType value="1"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gsubalternate.ttx.GSUB b/Tests/mtiLib/data/mti/gsubalternate.ttx.GSUB
index 86b0b731b..7762c621c 100644
--- a/Tests/mtiLib/data/mti/gsubalternate.ttx.GSUB
+++ b/Tests/mtiLib/data/mti/gsubalternate.ttx.GSUB
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- 27:  -->
     <Lookup index="0">
       <LookupType value="3"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gsubligature.ttx.GSUB b/Tests/mtiLib/data/mti/gsubligature.ttx.GSUB
index 26c88c81c..5ad201845 100644
--- a/Tests/mtiLib/data/mti/gsubligature.ttx.GSUB
+++ b/Tests/mtiLib/data/mti/gsubligature.ttx.GSUB
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- latinLigatures:  -->
     <Lookup index="0">
       <LookupType value="4"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gsubmultiple.ttx.GSUB b/Tests/mtiLib/data/mti/gsubmultiple.ttx.GSUB
index 5bedfba6a..72eefb87c 100644
--- a/Tests/mtiLib/data/mti/gsubmultiple.ttx.GSUB
+++ b/Tests/mtiLib/data/mti/gsubmultiple.ttx.GSUB
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- replace-akhand-telugu:  -->
     <Lookup index="0">
       <LookupType value="2"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/gsubreversechanined.ttx.GSUB b/Tests/mtiLib/data/mti/gsubreversechanined.ttx.GSUB
index d705af53d..87412eaec 100644
--- a/Tests/mtiLib/data/mti/gsubreversechanined.ttx.GSUB
+++ b/Tests/mtiLib/data/mti/gsubreversechanined.ttx.GSUB
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- arabicReverse:  -->
     <Lookup index="0">
       <LookupType value="8"/>
       <LookupFlag value="9"/><!-- rightToLeft ignoreMarks -->
diff --git a/Tests/mtiLib/data/mti/gsubsingle.ttx.GSUB b/Tests/mtiLib/data/mti/gsubsingle.ttx.GSUB
index dc6a29501..adc3ba524 100644
--- a/Tests/mtiLib/data/mti/gsubsingle.ttx.GSUB
+++ b/Tests/mtiLib/data/mti/gsubsingle.ttx.GSUB
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- alt-fractions:  -->
     <Lookup index="0">
       <LookupType value="1"/>
       <LookupFlag value="0"/>
diff --git a/Tests/mtiLib/data/mti/mark-to-ligature.ttx.GPOS b/Tests/mtiLib/data/mti/mark-to-ligature.ttx.GPOS
index b5f275eb0..f723670c0 100644
--- a/Tests/mtiLib/data/mti/mark-to-ligature.ttx.GPOS
+++ b/Tests/mtiLib/data/mti/mark-to-ligature.ttx.GPOS
@@ -3,6 +3,7 @@
   <Version value="0x00010000"/>
   <LookupList>
     <!-- LookupCount=1 -->
+    <!-- LigMk0:  -->
     <Lookup index="0">
       <LookupType value="5"/>
       <LookupFlag value="0"/>