@Scrabble96 which browser?

That tool you used is relying on PhantomJS. I'm not sure if PhantomJS supports webcomponents or if the polyfill (most of the errors come from there) is not compatible with PhantomJS...

@dgrammatiko
Thank you. I tried using both Firefox and Chrome. Same result, but, as you say, it might be an issue with PhantomJS.

But would that explain the 'errors' it found in joomla-alert-es5.min.js as well?

Of course, being in the min file they are all in line one line so I haven't a hope of tracking the errors down myself.

Enable debug to load non-minified files.

@Scrabble96 so here is the deal:

• The errors from /media/vendor/webcomponentsjs/js/webcomponents-sd-ce-pf.min.js refer to a script we are depending from Google. This is the polyfill for the webcomponents for the browsers that don't support that technology natively. Safari, Firefox, Chrome, Opera, Samsung, Brave and every other browser based on Chromium are already natively supporting Web Components. The only ones that don't are IE11 and Edge (edge soon will also use Chromium so...). That said the problem I see is the PhantomJS or the tool that you used to get the analysis. FWIW the polyfill is already tested against all Joomla supported browsers.

• The errors from the alert.js. So that script is part of another repo https://github.com/joomla-projects/custom-elements (docs: https://joomla-projects.github.io/custom-elements/#/) and up till sometime ago it had automated tests against all Joomla 4 supported browsers. FWIW that repo is under heavy refactoring atm but before any new code is published we need to fix all the Custom Elements in this repo. Unfortunately this is a task for me so things progressing slowly because I have limited spare time...

All and all I'll say ignore this tool's errors...

@dgrammatiko
Thank you, and sorry to take up so much of your precious time.

I will do as you say, ignore it and will now close this issue.