Find out why attributes are still arrays in theme

Problem/Motivation

Currently the attributes, like class for example, in the render arrays, are still regular array. Drupal has the Attribute class for this and should auto-convert it, see

• https://www.drupal.org/project/drupal/issues/2511154 →
• https://www.drupal.org/docs/8/theming-drupal-8/using-attributes-in-templ... →

still this doesn't happen for all cases, I guess because we have nested arrays there.

Perhaps we should convert them in https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21... or in the twig template?

What's best practice and which cases do we have to fix?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes