In presence of any strong acid, water acts as a proton acceptor. In presence of any strong base, water acts as a proton donor. Therefore, the ability of a strong acid to donate protons is determined by the proton accepting capacity of water and not on the acid. Hence, all strong acids show equal tendency to donate protons. Same thing happens with bases also. When the strong acids are taken in acetic acid, which is a very weak proton acceptor than water, the different acids donate protons to different extents. Therefore, the strengths of various strong acids can be compared in acetic acid.