https://www.wpg-tga.de/js/_essentials/devMode.js

1!function(){var e=function(e,o){console.log(e),e.style.outlineWidth="5px",e.style.outlineStyle="dashed",e.style.outlineColor=o};
2// CHECK ALT AND TITLE
3(n=document.querySelectorAll('a[title=""], a:not([title]), img[alt=""], img:not([alt]), iframe[frameborder]')).length&&notification.add({id:"devModeCSS",message:"Some of your elements are missing a TITLE- and/or ALT-attribute! Please check your browsers' console."}),console.group("Listing all elements with missing/empty attributes"),0===n.length&&console.log("Everything is fine.");for(var o=0;o<n.length;o++)e(n[o],"red");console.groupEnd();
4// CHECK BROKEN LINKS
5var n=document.querySelectorAll("a[href]"),t=!1;console.group("Listing all broken/strange anchors");var r=[];for(o=0;o<n.length;o++){var s;if(relativeURL=n[o].href.replace(window.location.origin,""),"/"!=relativeURL)(s=new XMLHttpRequest).open("GET",n[o].href,!1),s.send(null),200!==s.status&&(
6// console.log('Link Checker found a '+xhr.status+':');      
7t++,
8// highlight(elms[i], 'orange');
9r.push({status:s.status,element:n[o]}))}console.table(r),console.groupEnd(),t&&notification.add({id:"devModeLinkChecker",message:"Found strange links! Please check your browsers' console."}),(s=new XMLHttpRequest).addEventListener("readystatechange",function(e){if(4===this.readyState){var o=JSON.parse(this.response).messages;if(console.group("HTML Validation"),o.length>0)for(var n=0;n<o.length;n++){switch(o[n].type){case"error":case"non-document-error":color="red";break;case"info":color="yellow";break;default:color="inherit"}if("non-document-error"==o[n].type)console.info("It's not working, probably you're working on a test environment.");else{void 0!==o[n].firstLine?o[n].firstLine:o[n].lastLine;console.info("%c"+o[n].type+"%cline "+o[n].lastLine+": "+o[n].message,"color:"+color),void 0!==o[n].extract&&console.log("%c"+o[n].extract,"font-size:smaller")}}else console.log("%cGosh, no warning or error, seems to be a good source code.","color:limegreen");notification.add({id:"devModeValidator",message:"Validation complete. Please check your browsers' console."}),console.groupEnd()}}),s.open("GET","https://validator.w3.org/nu/?out=json&doc="+window.location.origin+"/?htmlminify=false"),s.send(null)}();