HTML writer: Allow "spanlike" classes to be combined.

Previously classes like "underline" and "marked" had to
be the first class in a span in order for the span to be
interpreted as a "ul" or "mark" element.  This commit allows
these special classes to be "stacked," e.g.
`[test]{.mark .underline}`; in addition, the special classes are no
longer required to come first in the list of classes.

See #8194 for context.

src/Text/Pandoc/Writers/HTML.hs

@@ -1389,15 +1389,28 @@ inlineToHtml opts inline = do
         -> inlineListToHtml opts ils >>= inDiv cls
 
     (Span (id',classes,kvs) ils) ->
-                        let spanLikeTag = case classes of
-                                (c:_) -> do
-                                  guard (c `Set.member` htmlSpanLikeElements)
-                                  pure $ customParent (textTag c)
-                                _   -> Nothing
-                        in case spanLikeTag of
-                            Just tag -> do
+                        let go Nothing c
+                             | c `Set.member` htmlSpanLikeElements
+                               = Just (customParent (textTag c), [])
+                             | c == "smallcaps"
+                               = Just (H.span ! A.class_ "smallcaps", [])
+                             | c == "underline"
+                               = Just (H.u, [])
+                             | otherwise = Nothing
+                            go (Just (t,cs)) c
+                             | c `Set.member` htmlSpanLikeElements
+                               = Just (t . customParent (textTag c), cs)
+                             | c == "smallcaps"
+                               = Just (t . (H.span ! A.class_ "smallcaps"), cs)
+                             | c == "underline"
+                               = Just (t . H.u, cs)
+                             | otherwise
+                               = Just (t, c:cs)
+                            spanLikeTags = foldl' go Nothing
+                        in case spanLikeTags classes of
+                            Just (tag, cs) -> do
                               h <- inlineListToHtml opts ils
-                              addAttrs opts (id',tail classes',kvs') $ tag h
+                              addAttrs opts (id',cs,kvs') $ tag h
                             Nothing -> do
                               h <- inlineListToHtml opts ils
                               addAttrs opts (id',classes',kvs') (H.span h)

test/command/nested-spanlike.md

@@ -0,0 +1,6 @@
+```
+% pandoc -f markdown -t html
+[test]{.foo .underline #bar .smallcaps .kbd}
+^D
+<p><u id="bar"><span class="smallcaps"><kbd>test</kbd></span></u></p>
+```