(def stop!
  (play-in-view! hero #(swap! page assoc :hero %) {:threshold 0.5}))

The browser's autoplay policy, the wait for the click, tap or key press
after which the page may play, and autoplay of a video while it's in
view.

The policy and the wait take these opts:

- :window, the window to ask and listen at instead of the page's, e.g.
  a stand-in in a test

Only Firefox has Autoplay Policy Detection, so elsewhere you find out by
calling media/play!, which answers {:refused :not-allowed} where the
browser won't play. play-when-allowed! plays then at the user's next
click, tap or key press.

Chrome and WebKit on iOS pause a muted video with the autoplay attribute
when it scrolls out of view, and play it again when it returns, but
leave alone a video that the page plays itself. play-in-view! does the
same for any video, in every engine, through media/play!:

    (def stop!
      (play-in-view! hero #(swap! page assoc :hero %) {:threshold 0.5}))

The allow attribute of an iframe that embeds a player from another site,
built from a map of the Permissions Policy features that the player may
use.

Without the attribute, a player from another site can't go fullscreen,
and its document reports document.fullscreenEnabled as false.

(def detach!
  (attach! audio "my-podcast-app"))

One player at a time across the tabs of an app, through Web Locks.

On a desktop, no engine stops the audio of one tab when another tab
starts, so two tabs of a podcast app play at once and race to save the
position. attach! holds a lock named for the app while the element
plays, and takes it from any other tab that holds it, which then pauses
its own element:

    (def detach!
      (attach! audio "my-podcast-app"))

The functions take these opts:

- :window, the window whose navigator holds the locks, instead of the
  page's, e.g. a stand-in in a test