"We" don't capture most of the value though. Most of the value goes to a tiny percentage of Americans at the very top and everyone else loses their jobs. (Ironically many of the rich I've spoken with agree, while it's often the middle and upper middle class who argue this point.)
The simplest thing to do would not be to try to measure regulation differential legally speaking, but to measure and tax two more direct things: pollution and cost-of-living-adjusted wage differential.
Pollution could be taxed at the aggregate national level, e.g. via CO2/$GDP differential. This is a direct measure of the energy and resource efficiency of an economy, and it likely correlates with other kinds of pollution as well.
https://en.wikipedia.org/wiki/List_of_countries_by_ratio_of_...
Imports from countries below the USA on that list would be levied an offshore externality tax.
Wages would actually be harder to assess since that gets complicated, but honestly the pollution tax alone would make a massive difference in leveling the playing field. Higher efficiency in that area also probably correlates positively with wages in more developed countries, so CO2/$GDP might be all you need to start with for a differential fair trade tax.
Some countries that are high on that list (low pollution) are there because they are largely undeveloped and don't use much energy. They'd get free trade with us that would help them develop, but as they developed they'd be incentivized to do so in a more sustainable and cleaner way to avoid incurring tariffs. Developing by screwing the environment as China and others have done would be disincentivized.
I'd also love to see human rights included, but that's harder to measure objectively and probably also correlates with wages and pollution-efficiency. Countries that don't give a damn about their population's rights probably also don't care if they huff heavy metal laden coal ash, and vice versa.