Is kissing the deceased by his relatives and friends a Sunnah, and is it permissible for his wife to kiss him?

It is permissible to kiss the deceased after death, whether they are relatives or not, due to authentic Sunnah to that effect. The Prophet, peace and blessings be upon him, kissed Uthman ibn Maz'un when he was deceased, and Abu Bakr Al-Siddiq, may Allah be pleased with him, kissed the Prophet, peace and blessings be upon him, after his death. There is no objection for a woman to kiss her husband after his death.