Store CSS hashed attribute in a variable

[Rich-Harris]

A nice easy win — currently if you have a component with CSS, top-level elements get this treatment:

setAttribute( div, 'svelte-3599223440', '' );

If there's more than one top-level element, it would make more sense to do this instead:

var cssAttr = 'svelte-3599223440';
// ...
setAttribute( div, cssAttr, '' );

Or even

function encapsulateStyles ( node ) {
  div.setAttribute( 'svelte-3599223440', '' );
}
// ...
encapsulateStyles( div );