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The Light DOM approach keeps native form submission, but the formula/result swap seems worth an accessibility check. Are both inputs consistently labelled, and can screen-reader users review the calculated value without losing the original formula context?

I had to implement this at work a couple of years ago, it was actually quite fun to build That one also had a *totally unnecessary* small floating numpad showing up below the input field on focus just to highlight that it accepted formulas lol

Does it act differently if the user puts an equals sign at the start or end of the formula?

No. The input does not accept the = sign. The whole expression you enter is evaluated.

My initial expectation was that = at the start would opt into evaluation, whereas = at the end would either replace the formula by its result or work as a comparison operator.

I wonder if this would look nicer using something like what Apple Notes does. So that the calculation is always shown but the result is rendered after. Downside would be that the form `value` wouldn't work (could use a separate hidden input)? And maybe OpaqueRange to highlight the result?