Taiwan, Japan, Singapore. Only countries you can safely drink tap water in Asia AFAIK.

Also South Korea, Brunei and Hong Kong (apparently).

But I have friends who rinse their mouth, brush with tap water in Vietnam and haven't gotten sick. It obviously varies a lot by which city (or even within the city), but the water is tested and obviously highly chlorinated (by smell).

I mean, I wouldn't drink it regularly because I don't think they look for non-biologic toxins like metals.

I've rinsed my mouth in most countries in SEA. But never drink it. In Taiwan you can drink tap water but vast majority of people boil it because apparently alot of the pipes in homes are old so they boil it first. Sometimes I'm lazy and don't want warm water so I just fill my bottle with tap water.