Different loading hints:

• async resource is loaded when the browser can
• defer resource is loaded when the DOM is parsed (less priority than async)
• rel=preload load the resource before the parsing and everything else
• fetchpriority with value high, low and auto. They are hints.
• rel=preconnect indicates that a connexion has to be established ASAP.
• rel=dns-prefetch resolves the domain name without fetching the resource.
• loading attribute determine when an image is loaded depending of its value:
  • eager: the image is loaded immediately
  • lazy: the loading is delayed until the user scroll to it.