Some Basic HTML, CSS, and JavaScript

gistfile1.txt

HTML:

<button id="b1" onclick="click()">Hi!</button>

JS:

function click() {
  alert("Hi!");
}

CSS:

#b1 {
  background-color: black;
}

Instead, here's the concept of how this would look in RWeb:

p=Page.new do
  title "Hi"
  Center.new(Button.new do
    text "Hi!"
    onclick {AlertBox.new("Hi!").displayModal}
  end)
end