The comma first is not with language design, it's a convention from FP langs (at least Haskell).

>What if you want to change the first item of a list?

    Html.program
            { view = view
            , update = update
            }

to

    Html.program
            { view = newView
            , update = update
            }

Leading comma vs allow a non-terminating comma both leads to the same diff of just the one line.

I mean, there are quite a bunch of languages that don't support non-terminating commas, which is really a bit of parsing that hides intent.

Other than that, bashing a language for having the convention (that is not forced by the language but by... convention) is quite ridiculous. The HTML comment is a valid point to raise, but there is quite a good reason for having it be native Elm, which brings a ton more benefits than having JSX'esgue syntax.