diff --git a/src/calibre/ebooks/oeb/polish/utils.py b/src/calibre/ebooks/oeb/polish/utils.py
index 3bfc465ec3..9c976b0b3c 100644
--- a/src/calibre/ebooks/oeb/polish/utils.py
+++ b/src/calibre/ebooks/oeb/polish/utils.py
@@ -14,6 +14,13 @@ from calibre import guess_type as _guess_type
 def guess_type(x):
     return _guess_type(x)[0] or 'application/octet-stream'
 
+def setup_cssutils_serialization(tab_width=2):
+    import cssutils
+    prefs = cssutils.ser.prefs
+    prefs.indent = tab_width * ' '
+    prefs.indentClosingBrace = False
+    prefs.omitLastSemicolon = False
+
 class PositionFinder(object):
 
     def __init__(self, raw):
diff --git a/src/calibre/gui2/tweak_book/boss.py b/src/calibre/gui2/tweak_book/boss.py
index 1f755dc186..45e14dcf9f 100644
--- a/src/calibre/gui2/tweak_book/boss.py
+++ b/src/calibre/gui2/tweak_book/boss.py
@@ -24,7 +24,7 @@ from calibre.ebooks.oeb.polish.pretty import fix_all_html, pretty_all
 from calibre.ebooks.oeb.polish.replace import rename_files, replace_file, get_recommended_folders, rationalize_folders
 from calibre.ebooks.oeb.polish.split import split, merge, AbortError, multisplit
 from calibre.ebooks.oeb.polish.toc import remove_names_from_toc, find_existing_toc
-from calibre.ebooks.oeb.polish.utils import link_stylesheets
+from calibre.ebooks.oeb.polish.utils import link_stylesheets, setup_cssutils_serialization as scs
 from calibre.gui2 import error_dialog, choose_files, question_dialog, info_dialog, choose_save_file
 from calibre.gui2.dialogs.confirm_delete import confirm
 from calibre.gui2.tweak_book import set_current_container, current_container, tprefs, actions, editors
@@ -54,11 +54,7 @@ def get_container(*args, **kwargs):
     return container
 
 def setup_cssutils_serialization():
-    import cssutils
-    prefs = cssutils.ser.prefs
-    prefs.indent = tprefs['editor_tab_stop_width'] * ' '
-    prefs.indentClosingBrace = False
-    prefs.omitLastSemicolon = False
+    scs(tprefs['editor_tab_stop_width'])
 
 class BusyCursor(object):
 
diff --git a/src/calibre/gui2/tweak_book/diff/main.py b/src/calibre/gui2/tweak_book/diff/main.py
index 81f2603551..db9c33d3f6 100644
--- a/src/calibre/gui2/tweak_book/diff/main.py
+++ b/src/calibre/gui2/tweak_book/diff/main.py
@@ -257,7 +257,7 @@ class Diff(Dialog):
         for i in (3, 5, 10, 50):
             cm.addAction(_('Show %d lines of context') % i, partial(self.change_context, i))
         cm.addAction(_('Show all text'), partial(self.change_context, None))
-        m.addAction(_('Beautify HTML/XML files before comparing them'), partial(self.change_beautify, True))
+        m.addAction(_('Beautify files before comparing them'), partial(self.change_beautify, True))
         m.addMenu(cm)
         l.addWidget(b, l.rowCount() - 1, l.columnCount(), 1, 1)
 
diff --git a/src/calibre/gui2/tweak_book/diff/view.py b/src/calibre/gui2/tweak_book/diff/view.py
index dcbdc85593..9dc377b85e 100644
--- a/src/calibre/gui2/tweak_book/diff/view.py
+++ b/src/calibre/gui2/tweak_book/diff/view.py
@@ -53,6 +53,19 @@ def beautify_text(raw, syntax):
     if syntax == 'xml':
         root = etree.fromstring(strip_encoding_declarations(raw))
         pretty_xml_tree(root)
+    elif syntax == 'css':
+        import logging
+        from calibre.ebooks.oeb.base import serialize, _css_logger
+        from calibre.ebooks.oeb.polish.utils import setup_cssutils_serialization
+        from cssutils import CSSParser, log
+        setup_cssutils_serialization(tprefs['editor_tab_stop_width'])
+        log.setLevel(logging.WARN)
+        log.raiseExceptions = False
+        parser = CSSParser(loglevel=logging.WARNING,
+                           # We dont care about @import rules
+                           fetcher=lambda x: (None, None), log=_css_logger)
+        data = parser.parseString(raw, href='<string>', validate=False)
+        return serialize(data, 'text/css')
     else:
         root = parse(raw, line_numbers=False)
         pretty_html_tree(None, root)
@@ -668,7 +681,7 @@ class DiffSplit(QSplitter):  # {{{
     def add_text_diff(self, left_text, right_text, context, syntax, beautify=False):
         left_text = unicodedata.normalize('NFC', left_text)
         right_text = unicodedata.normalize('NFC', right_text)
-        if beautify and syntax in {'xml', 'html'}:
+        if beautify and syntax in {'xml', 'html', 'css'}:
             left_text, right_text = beautify_text(left_text, syntax), beautify_text(right_text, syntax)
         left_lines = self.left_lines = left_text.splitlines()
         right_lines = self.right_lines = right_text.splitlines()