HTML -> djot introduces a redundant attribute on header

[jgm]

pandoc -f html -t djot
<h2 class="c1">Test</h2>
 ^D
{#test .c1}
{.c1}
## Test

[jgm]

This happens with any attribute other than id. And, oddly, it only affects headers, not e.g. divs or code blocks.

[jgm]

OK, here's why. The intermediate Djot Node structure produced is

Many
  { unMany =
      fromList
        [ Node
            NoPos
            (Attr [ ( "class" , "c" ) ])
            (Section
               Many
                 { unMany =
                     fromList
                       [ Node
                           NoPos
                           (Attr [ ( "class" , "c" ) ])
                           (Heading
                              1 Many { unMany = fromList [ Node NoPos (Attr []) (Str "hi") ] })
                       ]
                 })
        ]
  }

So, the first {.c} is from the Section, and the second from the Heading. That should give an idea how to fix this.

[jgm]

Internally makeSections is used to transform the AST produced by the HTML reader into an AST with "section" Divs. The classes (and some other attributes, but not id) are duplicated between the Header and the enclosing section Div. We ought to remove the duplicates before passing this off to the Djot writer.

[jgm]

A more robust solution might be to modify the Djot writer in djoths so that redundant attributes are removed before attributes are rendered.