An act of war, which the US would respond to with a freedom-of-navigation exercise backed up by the full might of US-PACOM. China would lose that standoff, which is why they're unlikely to do it unless they were confident of US non-involvement.
China is still modernising. They're outproducing the US in terms of new equipment, but flows aren't stocks, and the US also has decades of refinement and experience to draw on in comparison to China's zilch.
Don't get me wrong: the trend is towards Chinese dominance, but I don't think we're close yet. China would have to throw everything at Taiwan, and it would turn into a shooting gallery.
An act of war, which the US would respond to with a freedom-of-navigation exercise backed up by the full might of US-PACOM. China would lose that standoff, which is why they're unlikely to do it unless they were confident of US non-involvement.