HTML writer: Unwrap "wrapper" divs.

Some of the readers (e.g. djot) add "wrapper" divs to hold attributes
for elements that have no slot for attributes in the pandoc AST. With
this change, the HTML reader "unwraps" these wrappers so that the
attributes go on the intended elements.

Closes #11014.

src/Text/Pandoc/Writers/HTML.hs

@@ -845,6 +845,11 @@ blockToHtmlInner opts (Div (ident, "section":dclasses, dkvs)
                      if null innerSecs
                         then mempty
                         else nl <> innerContents
+blockToHtmlInner opts (Div (ident, classes, kvs) [b])
+  | Just "1" <- lookup "wrapper" kvs
+    -- unwrap "wrapper" div, putting attr on child
+  = blockToHtmlInner opts b >>=
+      addAttrs opts (ident, classes, [(k,v) | (k,v) <- kvs, k /= "wrapper"])
 blockToHtmlInner opts (Div attr@(ident, classes, kvs') bs) = do
   html5 <- gets stHtml5
   slideVariant <- gets stSlideVariant

test/command/11014.md

@@ -0,0 +1,9 @@
+```
+% pandoc -f djot -t html
+{.foo}
+- bar
+^D
+<ul class="foo">
+<li>bar</li>
+</ul>
+```