Thanks! I should have realised that a solution for this could be implemented in JavaScript as well, allowing it to run directly in the web browser. Here is my translation of my earlier Python program to JavaScript:

  let lo = 0, hi = dictionary.length - 1
  const answer = document.getElementById('guess')
  while (document.getElementsByClassName('correct').length === 0) {
    const mid = Math.floor(lo + (hi - lo) / 2)
    answer.value = dictionary[mid]
    guessWord()
    if (answer.placeholder.indexOf('after') !== -1) {
      lo = mid + 1
    } else {
      hi = mid - 1
    }
  }

This solution is quite similar to yours. Thanks for this nice idea!