I propose that eTLD+1 should be exposed to web content through location‍.site and new URL('…').site.

Webapps shouldn't have to embed the Public Suffix List (currently 222KB!) to lookup eTLD+1 when our browsers already have a copy.

API examples:

location.href === 'https://github.com/whatwg/url/issues/528'
location.site === 'github.com'

new URL('https://subdomain.google.co.uk').site === 'google.co.uk'