If you are a registered business and you show your online revenue to the gov, then yes you could charge tax (or you could pay it on behalf of the client).
My friend has his online business registered with the gov and he files his taxes every year. He does not collect taxes on his ad revenue. He simply charges a flat rate. If you do not want to pay the client's taxes, then sure you can charge it. However, I never came across someone who was charging a tax online.
Edit: I live in Canada and so does my friend.