img decoding attribute
> Decoding of large images can block the main thread for hundreds of milliseconds or more, interrupting fluid animations and user interaction. Currently, there's no way for a web author to specify that they want an image to be decoded asynchronously, so there are scenarios where it is impossible to avoid UI stalls.
Thus the new `decoding` attribute for the `img` element, with possible values:
* `sync`: Indicates a preference to decode this image synchronously for atomic presentation with other content.
* async: Indicates a preference to decode this image asynchronously to avoid delaying presentation of other content.
* auto: Indicates no preference in decoding mode (the default).
Background discussion: https://github.com/whatwg/html/issues/1920