HTML writer: use numeric character references with --ascii.

Previously we used named character references with html5 output.
But these aren't valid XML, and we aim to produce html5 that is
also valid XHTML (polyglot markup).  (This is also needed for
epub3.)

Closes #5718.

src/Text/Pandoc/Writers/HTML.hs

@@ -59,8 +59,7 @@ import Text.DocTemplates (Context(..))
 import Text.Pandoc.Walk
 import Text.Pandoc.Writers.Math
 import Text.Pandoc.Writers.Shared
-import Text.Pandoc.XML (escapeStringForXML, fromEntities,
-                        toEntities, toHtml5Entities)
+import Text.Pandoc.XML (escapeStringForXML, fromEntities, toEntities)
 #if MIN_VERSION_blaze_markup(0,6,3)
 #else
 import Text.Blaze.Internal (preEscapedString, preEscapedText)
@@ -207,10 +206,7 @@ writeHtmlString' :: PandocMonad m
 writeHtmlString' st opts d = do
   (body, context) <- evalStateT (pandocToHtml opts d) st
   (if writerPreferAscii opts
-      then
-        if stHtml5 st
-           then toHtml5Entities
-           else toEntities
+      then toEntities
       else id) <$>
     case writerTemplate opts of
        Nothing -> return $ renderHtml' body

test/command/ascii.md

@@ -2,7 +2,7 @@
 pandoc -t html --ascii
 äéıå
 ^D
-<p>&auml;&eacute;&imath;&aring;</p>
+<p>&#228;&#233;&#305;&#229;</p>
 ```
 
 ```