Hyperbole encourages using the atomic-css package to factor styles with haskell functions. Here is how we might create reusable header and btn styles:
import Web.Atomic.CSS
import Web.Hyperbole

example = do
  col $ do
    el ~ h3 $ "My Page"
    el ~ btn $ "Hover Me"
 where
  header = bold
  h1 = header . fontSize 32
  h2 = header . fontSize 24
  h3 = header . fontSize 18

  btn =
    pad 10 . border 1 . pointer . hover (bold . border 2)
Note how we use hover to provide immediate feedback to the user without talking to the server
If an update changes the transition property of a view, it will automatically animate with CSS Transitions, avoiding having the server compute animation frames.
viewSmall :: View Animate ()
viewSmall = do
  col ~ gap 10 . transition 300 (Width 200) $ do
    el "Small"
    button Expand "Expand" ~ btn

viewBig :: View Animate ()
viewBig =
  col ~ gap 10 . transition 300 (Width 400) $ do
    el "Expanded"
    button Collapse "Collapse" ~ btn
For immediate feedback, create interactivity via Atomic CSS whenever possible.
Use whenLoading to provide feedback while an Action is being processed
viewLoaders :: Text -> View Loader ()
viewLoaders status = do
  col ~ gap 10 $ do
    row ~ gap 10 . whenLoading flexRow . display None $ do
      loadingBars
      el "Loading..."
    el ~ whenLoading (display None) $ text status
    button LoadSlow ~ btn . whenLoading (opacity 0.5) $ "Load Slow"
You can opt-out of Atomic CSS and use external classes with class_
page :: (Hyperbole :> es) => Page es '[Items]
page = do
  pure $ do
    -- you can choose to include a stylesheet only on pages
    -- that use it or load it globally in your document function
    stylesheet "external.css"
    hyper Items $ itemsView "one"

itemsView :: Text -> View Items ()
itemsView sel = do
  el @ class_ "parent" $ do
    item "one"
    item "two"
    item "three"
    item "four"
    item "five"
 where
  selected i =
    if sel == i
      then class_ "selected"
      else id

  item i =
    -- the class_ attribute MERGES classes if you set it more than once
    button (Select i) @ class_ "item" . selected i $ text i