Given the id attribute of a <form> tag, we can decode the current form values from the browser document and into a List of key values.

view model =
    -- form with `id` attribute
    form [ id "edituserform123" ]
        -- fields with `name` attribute
        []


update msg model =
    ( { model
        | formFields =
            -- decode model.document to obtain a list of
            -- form field values anytime
            model.document
                |> Json.Decode.decodeValue (NativeForm.decoder "edituserform123")
                |> Result.withDefault []
      }
    , Cmd.none
    )

We are returning a List instead of Dict because on form submit, duplicate names are preserved. So we are preserving them here too. If a Dict is desired, use the valuesDict helper function

    update msg model =
    ( { model
        | formFields =
            -- decode model.document to obtain a list of
            -- form field values anytime
            model.document
                |> Json.Decode.decodeValue (NativeForm.decoder "edituserform123")
+               |> Result.map NativeForm.valuesDict
                |> Result.withDefault []
    }
    , Cmd.none
    )

Values from form fields are either String or List String

e.g. values for input [ type_ "number" ] [] will still be a String since and is entirely up to your application to convert and validate it with String.toInt or String.toFloat

Given a list of key values, combine the values of duplicate keys

import Dict exposing (Dict)


valuesDict
    [ (1, OneValue "1")
    , (2, OneValue "a")
    , (2, ManyValues ["b","c"])
    , (3, ManyValues ["yes","no"])
    ]
--> Dict.fromList
-->     [ (1, OneValue "1")
-->     , (2, ManyValues ["a","b","c"])
-->     , (3, ManyValues ["yes","no"])
-->     ]

Given 2 Value, return a ManyValues

valuesAppend (OneValue "1") (OneValue "a")
--> ManyValues ["1","a"]


valuesAppend (OneValue "1") (ManyValues ["a","b"])
--> ManyValues ["1","a","b"]


valuesAppend (ManyValues ["1","2"]) (ManyValues ["a","b"])
--> ManyValues ["1","2","a","b"]


valuesAppend (ManyValues ["1","2"]) (OneValue "a")
--> ManyValues ["1","2","a"]

OneValue 3
|> oneMap ((+) 2)
--> OneValue 5


ManyValues [ 3 ]
|> oneMap ((+) 2)
--> ManyValues []

OneValue 42
|> oneWithDefault 3
--> 42


ManyValues [42]
|> oneWithDefault 3
--> 3

OneValue 3
|> manyMap ((++) [ 2 ])
--> ManyValues []


ManyValues [ 3 ]
|> manyMap ((++) [ 2 ])
--> ManyValues [ 2, 3 ]

OneValue 42
|> manyWithDefault [3]
--> [3]


ManyValues [42]
|> manyWithDefault [3]
--> [42]