Great.
Remains one aspect: the Module Class. What do we do with that?

Not sure I understand what you mean. Is it missing from the output?

Is it missing from the output?

As far as I see , yes.

I think it comes from
<jdoc:include type="modules" name="banner" style="xhtml" />

Cardgrey and default chromes contain <?php echo htmlspecialchars($params->get('moduleclass_sfx')

not xhtml

I've changed the style to html5 which appears to be the only chrome with no styling and adds the class. It appears default applies card styling to the module which is incorrect imo and possibly needs changing (separate issue/pr).

On ipad, will test later.
In j3, chromes are defined in system.
Except for the specific chromes which are defined in the template itself, do you have an idea where are defined html5 and xhtml in j4 ?

System chromes are in ../layouts/chromes/

I have tested this item ✅ successfully on bdcce04

Looks fine here.

we get <div class="moduletable mybannerclass"> which is fine.

@ciar4n
Looks like we have also another xhtml style in error.php

<?php if ($this->countModules('banner')) : ?>
		<div class="grid-child container-banner">
			<jdoc:include type="modules" name="banner" style="xhtml" />
		</div>

I have tested this item ✅ successfully on aee0cc4

Tested hours before, but after applying Patch, I got error 404 and: Call to undefined method Joomla\CMS\WebAsset\WebAssetManager::usePreset()
Then I found: brianteeman@7fa8d42
My site was from oct2019, therefore checked above files. Finally installed now completely new nightly build from today.

I have tested this item ✅ successfully on aee0cc4

RTC

tks

Sorry, I know the original issue was posted by me, but I'm unable to test this; I'm waiting for a new computer :-( which should arrive sometime next week.