Main.elm

import Browser
import Html exposing (Attribute, Html, button, div, node, p, text)
import Html.Attributes exposing (classList)
import Html.Events exposing (onClick)

type alias Model = { message : String , flush : Bool }

main =
  Browser.sandbox
    { init = { message = "" , flush = False }
    , update = update
    , view = view
    }

type Msg = Increment | Flush

update : Msg -> Model -> Model
update msg model =
  case msg of
    Increment ->
      { model | message = model.message ++ "💩" }
    Flush ->
      { model | flush = True }

marquee : List (Attribute msg) -> List (Html msg) -> Html msg
marquee attributes children =
  node "marquee" attributes children

view : Model -> Html Msg
view model =
  div
    []
    [ (if model.flush then marquee else p) [] [ text model.message ]
    , button [ onClick Increment ] [ text "増える" ]
    , button [ onClick Flush ] [ text "流す" ]
    ]

sample.sh

npm init -y
npm install elm --save-dev
$(npm bin elm)/elm init
$(npm bin elm)/elm make Main.elm