diff --git a/Text/Hamlet.hs b/Text/Hamlet.hs
--- a/Text/Hamlet.hs
+++ b/Text/Hamlet.hs
@@ -205,7 +205,7 @@
   where
     go :: (Deref, [Doc]) -> Q Exp
     go (d, docs) = do
-        let d' = derefToExp scope d
+        let d' = derefToExp ((specialOrIdent, VarE 'or):scope) d
         docs' <- docsToExp env hr scope docs
         return $ TupE [d', docs']
 docToExp env hr scope (DocCase deref cases) = do
diff --git a/Text/Hamlet/Parse.hs b/Text/Hamlet/Parse.hs
--- a/Text/Hamlet/Parse.hs
+++ b/Text/Hamlet/Parse.hs
@@ -12,6 +12,7 @@
     , CloseStyle (..)
     , Binding (..)
     , NewlineStyle (..)
+    , specialOrIdent
     )
     where
 
@@ -24,7 +25,7 @@
 import Text.ParserCombinators.Parsec hiding (Line)
 import Data.Set (Set)
 import qualified Data.Set as Set
-import Data.Maybe (mapMaybe, fromMaybe)
+import Data.Maybe (mapMaybe, fromMaybe, isNothing)
 
 data Result v = Error String | Ok v
     deriving (Show, Eq, Read, Data, Typeable)
@@ -464,7 +465,7 @@
     let attrs' =
             case clazzes of
               [] -> map attrFix noclass
-              _ -> (Nothing, "class", map (second Just) clazzes)
+              _ -> (testIncludeClazzes clazzes, "class", map (second Just) clazzes)
                        : map attrFix noclass
     let closeStyle =
             if not (null content) || not (null inside)
@@ -660,3 +661,24 @@
 
 spaceTabs :: Parser String
 spaceTabs = many $ oneOf " \t"
+
+-- | When using conditional classes, it will often be a single class, e.g.:
+--
+-- > <div :isHome:.homepage>
+--
+-- If isHome is False, we do not want any class attribute to be present.
+-- However, due to combining multiple classes together, the most obvious
+-- implementation would produce a class="". The purpose of this function is to
+-- work around that. It does so by checking if all the classes on this tag are
+-- optional. If so, it will only include the class attribute if at least one
+-- conditional is true.
+testIncludeClazzes :: [(Maybe Deref, [Content])] -> Maybe Deref
+testIncludeClazzes cs
+    | any (isNothing . fst) cs = Nothing
+    | otherwise = Just $ DerefBranch (DerefIdent specialOrIdent) $ DerefList $ mapMaybe fst cs
+
+-- | This funny hack is to allow us to refer to the 'or' function without
+-- requiring the user to have it in scope. See how this function is used in
+-- Text.Hamlet.
+specialOrIdent :: Ident
+specialOrIdent = Ident "__or__hamlet__special"
diff --git a/Text/Hamlet/RT.hs b/Text/Hamlet/RT.hs
--- a/Text/Hamlet/RT.hs
+++ b/Text/Hamlet/RT.hs
@@ -101,6 +101,11 @@
         els' <- maybe (return []) (mapM convert) els
         return $ SDCond conds' els'
       where
+        -- | See the comments in Text.Hamlet.Parse.testIncludeClazzes. The conditional
+        -- added there doesn't work for runtime Hamlet, so we remove it here.
+        go (DerefBranch (DerefIdent x) _, docs') | x == specialOrIdent = do
+            docs'' <- mapM convert docs'
+            return (["True"], docs'')
         go (deref, docs') = do
             deref' <- flattenDeref' x deref
             docs'' <- mapM convert docs'
@@ -176,6 +181,7 @@
             => [String] -> [String] -> HamletMap url -> m (HamletData url)
     lookup' orig k m =
         case lookup k m of
+            Nothing | k == ["True"] -> return $ HDBool True
             Nothing -> fa $ showName orig ++ ": not found"
             Just x -> return x
 
diff --git a/hamlet.cabal b/hamlet.cabal
--- a/hamlet.cabal
+++ b/hamlet.cabal
@@ -1,5 +1,5 @@
 name:            hamlet
-version:         1.1.6.3
+version:         1.1.6.4
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>
diff --git a/test/HamletTest.hs b/test/HamletTest.hs
--- a/test/HamletTest.hs
+++ b/test/HamletTest.hs
@@ -439,6 +439,12 @@
     $of _
         <p>Many
 |]
+    it "optional and missing classes" $
+        helper "<i>foo</i>\n" [hamlet|<i :False:.not-present>foo|]
+    it "multiple optional and missing classes" $
+        helper "<i>foo</i>\n" [hamlet|<i :False:.not-present :False:.also-not-here>foo|]
+    it "optional and present classes" $
+        helper "<i class=\"present\">foo</i>\n" [hamlet|<i :False:.not-present :True:.present>foo|]
 
 data Pair = Pair String Int