diff --git a/src/calibre/ebooks/oeb/polish/cascade.py b/src/calibre/ebooks/oeb/polish/cascade.py
index 35baa9af0b..677c693a67 100644
--- a/src/calibre/ebooks/oeb/polish/cascade.py
+++ b/src/calibre/ebooks/oeb/polish/cascade.py
@@ -63,7 +63,7 @@ def iterrules(container, rules, sheet_name, media_rule_ok=media_allowed, rule_in
             num = next(rule_index_counter)
             yield rule, sheet_name, num
 
-StyleDeclaration = namedtuple('StyleDeclaration', 'index sheet_name declaration pseudo_element')
+StyleDeclaration = namedtuple('StyleDeclaration', 'index declaration pseudo_element')
 Specificity = namedtuple('Specificity', 'is_style num_id num_class num_elem rule_index')
 
 def specificity(rule_index, selector, is_style=0):
@@ -78,29 +78,39 @@ def iterdeclaration(decl):
         else:
             for k, v in n(p.name, p.propertyValue).iteritems():
                 yield Property(k, v, p.literalpriority)
+class Values(tuple):
 
-def normalize_style_declaration(decl):
+    ''' A tuple of `cssutils.css.Value ` (and its subclasses) objects. Also has a
+    `sheet_name` attribute that is the canonical name relative to which URLs
+    for this property should be resolved. '''
+
+    def __new__(typ, pv, sheet_name=None, priority=''):
+        ans = tuple.__new__(typ, pv)
+        ans.sheet_name = sheet_name
+        ans.is_important = priority == 'important'
+        return ans
+
+def normalize_style_declaration(decl, sheet_name):
     ans = {}
     for prop in iterdeclaration(decl):
-        ans[prop.name] = (prop.propertyValue, prop.priority)
+        ans[prop.name] = Values(prop.propertyValue, sheet_name, prop.priority)
     return ans
 
 def resolve_declarations(decls):
     property_names = set()
-    for decl in decls:
-        property_names |= set(decl)
+    for d in decls:
+        property_names |= set(d.declaration)
     ans = {}
     for name in property_names:
         first_val = None
         for decl in decls:
-            x = decl.get(name)
+            x = decl.declaration.get(name)
             if x is not None:
-                val, priority = x
-                if priority == 'important':
-                    first_val = val
+                if x.is_important:
+                    first_val = x
                     break
                 if first_val is None:
-                    first_val = val
+                    first_val = x
         ans[name] = first_val
     return ans
 
@@ -122,13 +132,13 @@ def resolve_styles(container, name):
                     container.log.error('Ignoring CSS rule with invalid selector: %r (%s)' % (text, as_unicode(err)))
                     continue
                 m = pseudo_pat.search(text)
-                style = normalize_style_declaration(rule.style)
+                style = normalize_style_declaration(rule.style, sheet_name)
                 if m is None:
                     for elem in matches:
-                        style_map[elem].append(StyleDeclaration(specificity(rule_index, selector), sheet_name, style, None))
+                        style_map[elem].append(StyleDeclaration(specificity(rule_index, selector), style, None))
                 else:
                     for elem in matches:
-                        pseudo_style_map[elem].append(StyleDeclaration(specificity(rule_index, selector), sheet_name, style, m.group(1)))
+                        pseudo_style_map[elem].append(StyleDeclaration(specificity(rule_index, selector), style, m.group(1)))
 
     process_sheet(html_css_stylesheet(), 'user-agent.css')
 
@@ -158,7 +168,7 @@ def resolve_styles(container, name):
         text = elem.get('style')
         if text:
             style = container.parse_css(text, is_declaration=True)
-            style_map[elem].append(StyleDeclaration(Specificity(1, 0, 0, 0, 0), name, normalize_style_declaration(style), None))
+            style_map[elem].append(StyleDeclaration(Specificity(1, 0, 0, 0, 0), normalize_style_declaration(style, name), None))
 
     for l in (style_map, pseudo_style_map):
         for x in l.itervalues():
@@ -175,14 +185,14 @@ def defvals():
     global _defvals
     if _defvals is None:
         u = type('')
-        _defvals = {k:Property(k, u(val)).propertyValue for k, val in DEFAULTS.iteritems()}
+        _defvals = {k:Values(Property(k, u(val)).propertyValue) for k, val in DEFAULTS.iteritems()}
     return _defvals
 
 def get_resolved_property(elem, name, style_map):
     ''' Given a `style_map` previously generated by :func:`resolve_styles()` and
     a property `name`, returns the effective value of that property for the
     specified element. Handles inheritance and CSS cascading rules. Returns
-    an instance of `cssutils.css.PropertyValue`. If the property was never set and
+    an instance of :class:`Values`. If the property was never set and
     is not a known property, then it will return None. '''
 
     inheritable = name in INHERITED