altChecker.js

(function() {
    'use strict';
    function addCssRule(rule) {
        var head, style; 
        head = document.querySelector('head'); 
        if(!head) { 
            return;
        }
        style = document.createElement('style');
        style.type='text/css';
        style.innerHTML=rule;
        head.appendChild(style);
    }
    
    addCssRule("img[alt=''],img:not([alt]){filter: blur(10px) !important;outline: 10px solid !important; }"); 
})();

Not sure why that's happening - this was copied and pasted right from the script running on my machine. Do you have other scripts running through Tampermonkey? Could be a conflict somewhere...

No monkey scripts here, but I just re-installed script and it works.

Also this might catch both empty alt and missing alt

 img[alt=””], img:not([alt]) {
    ….
 }