Replace images that have alt attributes with their text.

img-alt-reveal-marklet.js

Array.from(document.querySelectorAll('img[alt]:not([alt=""])')).forEach($el => {
    const $div = document.createElement('div');
    $div.style.display = 'inline-block';
    const {width, height} = $el.getBoundingClientRect();
    $div.style.width = width;
    $div.style.height = height;
    $div.innerText = $el.getAttribute('alt');
    $el.parentNode.replaceChild($div, $el);
});