Finage
Finage is a stock, currency, cryptocurrency, indices, and ETFs real-time & historical data provider

Current UK and US tax rates, allowances and thresholds.
Finance Clearly Tax Rates API is a free Finance API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
A free JSON endpoint with current UK and US tax rates. No key, no signup, no rate limit. Maintained by a chartered management accountant.
Details from financeclearly.com.

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Finage is a stock, currency, cryptocurrency, indices, and ETFs real-time & historical data provider
Official-source financials, segments, valuation, peers and prices for KR, US, JP, TW companies
The Finance Clearly Tax Rates API is provided by Finance Clearly (financeclearly.com). A free JSON endpoint with current UK and US tax rates. No key, no signup, no rate limit. Maintained by a chartered management accountant.
Yes. The Finance Clearly Tax Rates API is listed in the Finance category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Finance Clearly Tax Rates API requires no authentication, so you can start making requests without signing up.
Yes. The Finance Clearly Tax Rates API supports CORS, so front-end JavaScript can call it directly.
Yes. The Finance Clearly Tax Rates API is served over HTTPS, so it's safe to use in production.