Clear consequences have not, as near as I can tell, be spelled out yet. The gist is that US trade policy will stop privileging HK. US/HK trade seems to be something like $66 Billion a year, so this is potentially a major thing.
https://www.nytimes.com/2019/09/11/fashion/watches-hong-kong...
It remains to be seen if sanctioning China (partially) could bring about political change, as CCP is endorsed within China mostly for economic growth in the past three decades.
But it's not the point; The intention is reduce the capabilities of the country as a global influence and generate internal state turmoil.
Is that really relevant?
I mean, HKers are getting screwed by a massively totalitarian and oppressive regime. Why is the interest of a third party relevant to the Chinese regime's devastation of Hong Kong?
"U.S. Officially Declares That Hong Kong Is No Longer Autonomous
WASHINGTON—The State Department has officially determined that Hong Kong is no longer autonomous from China, Secretary of State Mike Pompeo said in a statement Wednesday that holds implications for the future of economic ties and could lead to sanctions against China.
The State Department is required by the Hong Kong Policy Act to assess the autonomy of the territory from China. It certified to Congress on Thursday that Hong Kong is no longer autonomous."