OTF subsetting basically works, but leaves behind invalid entries in the GPOS table, which should be ignored by most user agents

src/calibre/ebooks/conversion/plumber.py

@@ -210,9 +210,7 @@ OptionRecommendation(name='subset_embedded_fonts',
             'Subset all embedded fonts. Every embedded font is reduced '
             'to contain only the glyphs used in this document. This decreases '
             'the size of the font files. Useful if you are embedding a '
-            'particularly large font with lots of unused glyphs. Note that '
-            'subsetting is only supported for fonts that contain TrueType '
-            'outlines, not Postscript outlines.')
+            'particularly large font with lots of unused glyphs.')
         ),
 
 OptionRecommendation(name='linearize_tables',

src/calibre/gui2/convert/look_and_feel.ui

@@ -6,7 +6,7 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>655</width>
+    <width>699</width>
     <height>619</height>
    </rect>
   </property>
@@ -411,7 +411,7 @@
    <item row="6" column="3" colspan="2">
     <widget class="QCheckBox" name="opt_subset_embedded_fonts">
      <property name="text">
-      <string>&amp;Subset all embedded fonts</string>
+      <string>&amp;Subset all embedded fonts (Experimental)</string>
      </property>
     </widget>
    </item>

src/calibre/utils/fonts/sfnt/cff/writer.py

@@ -107,7 +107,7 @@ class Subset(object):
 
     def __init__(self, cff, keep_charnames):
         self.cff = cff
-        self.keep_charnames = keep_charnames
+        keep_charnames.add(b'.notdef')
 
         header = pack(b'>4B', 1, 0, 4, cff.offset_size)
 
@@ -127,7 +127,7 @@ class Subset(object):
             cname = self.cff.charset.safe_lookup(i)
             if cname in keep_charnames:
                 char_strings.append(self.cff.char_strings[i])
-                self.charname_map[cname] = i
+                self.charname_map[cname] = len(self.charname_map)
                 if i > 0: # .notdef is not included
                     charsets.append(cname)
 

src/calibre/utils/fonts/sfnt/subset.py

@@ -167,7 +167,7 @@ def main(args):
     with open(iff, 'rb') as f:
         orig = f.read()
 
-    chars = [x.strip() for x in chars.split(',')]
+    chars = [x for x in chars.split(',')]
     individual, ranges = set(), set()
 
     def not_single(c):