CSS preloaders
In shinydashboardPlus: Add More 'AdminLTE2' Components to 'shinydashboard'

library(bslib)
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

How to set up a preloader?

Pass the argument preloader to the dashboardPage() function. It expects a nested list containing all parameters necessary to waiter::waiterShowOnLoad. Please have a look to the {waiter} documentation for more details.

That's all!

card(
  shiny::tags$iframe(
    class = "html-fill-item", 
    src = "https://shinylive.io/r/app/#h=0&code=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", 
    height = "800", 
    width = "100%", 
    style = "border: 1px solid rgba(0,0,0,0.175); border-radius: .375rem;", 
    allowfullscreen = "", 
    allow = "autoplay", 
    `data-external` = "1"
  ),
  full_screen = TRUE,
  style = "margin: 0 auto; float: none;"
)