If with online you mean hosted by a company u can always get it back.

If it's a wallet where you control the keys, such as jaxx then it's gone

Tricky situation but the coins doesn't necessary have to be lost. Many web wallets have ways to,recover your funds, even if the password is lost, like blockchain. The best way is to contact the customer service, explain what happened and I'm sure there will be some kind of solution.

That person would use the recover password functionality that exists in many places. If you were questioning what happens if the owner's identity cannot be proven, that probably depends from provider to provider (maybe you can find it in their TOS, or something like that).

In general, funds would probably remain stuck until the wallet services shut down. At which point, yes, they would probably be lost for good.

Yeah it depends on the service. and The question came to my mind after looking at a notice of blockchain wallet. once when I was creating my blockchain it said that if I lost my wallet ID or password then I would not be able to recover my coins.

I think this problem can be solved in most of cases, It depend with which web wallet that person(x) use and the company. So, x can contact them and try to solve the problem with them by sharing his account information. due to that, he can recover his account and bitcoin. Anyways, I don't think this is a big problem.

You can forgot password to recover your account. However if there is no other way to have access on it, I think the BTC will stay forever unless the owner or administrator is aware on how to get it. That's why bitcoin is decreasing because we consider it as lost.

Just if an online wallet is just deducting some bitcoin from some wallet that they consider as a non-active wallet and have been left by the owner, it's still not their right to take it and it can be considered as a wrong action unless they're already having an agreement towards their client regarding this, therefore, the online wallet provider which have done this could be considered as an untrustworthy service provider, it's also a proof that the service providers still keeping sensitive data in their database.
And because of that, the bitcoin will remain forever in the wallet until the real owners have an access to get through and take their own BTC.

Most members are saying the same thought which is very true because this would depends on the service,asking for password recovery would be so troublesome if you cant provide proofs that you own that wallet and all web wallet provider do really have the seeds which there a possibility those bitcoins could be accessed again.

Well, since you're specifically referring to blockchain.info then they've already said it pretty clearly.
They claim to have no copy of user's password, so if you loose it, you better have backed-up your secret phrase/mnemonic.

If you do, then I think they'd be able to help you recover/generate the wallet. If not, then I'm not sure they'd accommodate other ways of verifying account ownership. If you were actively buying/selling from their exchange with your personal bank account/cards/etc, then they might be able to verify via the documents, but if you were just using their wallet, then that's hard.

So, make sure you always backup any/all mnemonics for your wallets.

Those bitcoins will not be lost but will remain untouched. The site in their agreement with the users will clearly state that the bitcoin in the wallets will remain untouched by the company except if necessary. Those bitcoins though not lost will be considered lost since they will be left untouched until the site is still running. But this will be a warning to all of us to really keep the password safe so we wont regret anything.