Prevent placeholder class to affect em

Problem/Motivation

core/lib/Drupal/Component/Render/FormattableMarkup.php::placeholderFormat() is using <em class="placeholder"> for "%" replacement in strings.

The "placeholder" class interfere with Bootstrap default CSS.

I already did some preprocess in src/HookHandler/PreprocessStatusMessages.php to remove the placeholder class but it is specific to the 'status_messages' theme key.

And placeholder replacement can happen anywhere.

Steps to reproduce

Install Gin Toolbar and/or use Layout Builder.

Proposed resolution

Either alter FormattableMarkup to remove the class directly here. I am not sure it will be possible as it is a static method called directly and not a service.

Also we only need to change one line in long method, not possible to override only one small method.

Other solution will be to provide an override of Bootstrap CSS to ensure its ".placeholder" CSS does not apply to em.placeholder.

Remaining tasks

Try either solutions.