Funny, most of those HK demonstrators would be too young to remember/be aware of the the 1989 Tiananmen Square massacre...
I don't know how it is in HK given their special status, but I know for the majority of China the government has gone to great length to make sure that if you ask someone on the street about it... they don't have a clue what you are talking about, or worse think it's a Western conspiracy that never happened.
Giving Hong Kong back to the Chinese was one of the biggest mistakes the West made, because it's massive economy was a huge boon (and still is) for the Chinese government. The people of HK should have been given a choice on what they wanted to do, and if they chose to stay independent or a part of the UK they should have been protected like Taiwan is.
Perhaps that would have lead to war with China, but one way or another that war is inevitable anyways... we should have fought it when we were the more powerful of the two sides. Now, it's just a matter of time before China beats us... if not with guns, by sheer economic and cultural power.