diff --git a/Changelog.yaml b/Changelog.yaml
index 87a03a470e..f2eee7dbef 100644
--- a/Changelog.yaml
+++ b/Changelog.yaml
@@ -39,6 +39,12 @@
     - title: "calibredb with output format stanza once again escapes URLs"
       tickets: [4084]
 
+    - title: Implement work around for a CSS bug in Adobe Digital Editions that causes it to crash
+      description: >
+            If the CSS contains the declaration font-size: medium instead of font-size: normal
+            Adobe Digital Editions will crash. calibre will now automatically replace medium with
+            normal.
+
     - title: "PML Output: Only create \\a and \\U tags for supported characters."
 
     - title: "PML Input: Allow for images in top-level bookname_image and images directories"
diff --git a/src/calibre/ebooks/oeb/base.py b/src/calibre/ebooks/oeb/base.py
index 20c8a3429f..b37f51c6b5 100644
--- a/src/calibre/ebooks/oeb/base.py
+++ b/src/calibre/ebooks/oeb/base.py
@@ -780,6 +780,8 @@ class Manifest(object):
             # In particular, it causes tostring to insert xmlns
             # declarations, which messes up the coercing logic
             idx = data.find('<html')
+            if idx == -1:
+                idx = data.find('<HTML')
             if idx > -1:
                 pre = data[:idx]
                 data = data[idx:]
diff --git a/src/calibre/ebooks/oeb/transforms/flatcss.py b/src/calibre/ebooks/oeb/transforms/flatcss.py
index fb32a1ccf9..4d21c33c83 100644
--- a/src/calibre/ebooks/oeb/transforms/flatcss.py
+++ b/src/calibre/ebooks/oeb/transforms/flatcss.py
@@ -229,6 +229,8 @@ class CSSFlattener(object):
         if 'bgcolor' in node.attrib:
             cssdict['background-color'] = node.attrib['bgcolor']
             del node.attrib['bgcolor']
+        if cssdict.get('font-weight', '').lower() == 'medium':
+            cssdict['font-weight'] = 'normal' # ADE chokes on font-weight medium
         if not self.context.disable_font_rescaling:
             _sbase = self.sbase if self.sbase is not None else \
                 self.context.source.fbase