I'd be surprised if any country responded militarily. It's not in the interest of anyone to risk WW3 over Taiwan, and China knows this. In contrast to Russia, they are probably competent enough to take Taiwan in a day or two before anyone can send significant aid.

There would be massive economic implications though. The most effective deterrence that western countries have are sanctions, and they can't let China invade without consequences to keep the deterrence effective (also for other countries), so they'd have to be used. Investment in China would probably be banned for decades to reduce reliance.

If hundreds of millions of Chinese fall back into poverty due to the invasion, that would be a serious threat for political stability in China.