Markdown reader: accept processing instructions as raw HTML.

Closes #4125.

src/Text/Pandoc/Readers/HTML.hs

@@ -1155,8 +1155,9 @@ htmlTag f = try $ do
   -- in XML element names
   let isNameChar c = isAlphaNum c || c == ':' || c == '-' || c == '_'
   let isName s = case s of
-                      []     -> False
-                      (c:cs) -> isLetter c && all isNameChar cs
+                      []      -> False
+                      ('?':_) -> True -- processing instruction
+                      (c:cs)  -> isLetter c && all isNameChar cs
 
   let endpos = if ln == 1
                   then setSourceColumn startpos

test/command/4125.md

@@ -0,0 +1,6 @@
+```
+% pandoc
+<?asciidoc-toc?>
+^D
+<?asciidoc-toc?>
+```