IANA maintains a list of TLDs in the root zone[0]. I presume browsers use this list, since it does cover all TLDs in >99% of situations.

Yes but local/corporate DNS servers can use any TLD they want...

	▲	MarsIronPI 2 days ago \| parent [-]
		Right, but browsers don't automatically recognize those, as far as I know. If you want to visit a site with a custom TLD I believe you have to prefix it with the protocol.