TYPO3 Core

When setting your own CKEditor preset RTE TSConfig configuration is lost. This result in not being able to configure CSS classes for LinkHandlers via TSConfig. I debugged this down to
\TYPO3\CMS\RteCKEditor\Controller\BrowseLinksController which depends on TSConfig-arrays retrieved by \TYPO3\CMS\Core\Configuration\Richtext.

Responsible for this behavior (losing the TSConfig configuration) is \TYPO3\CMS\Core\Configuration\Richtext->getConfiguration(). The whole configuration array is overwritten by the parsed custom preset YAML file.

A workaround is to emulate a TSConfig-like array in your YAML preset to add configuration which would usually be found in TSConfig (RTE.*, RTE.default):

imports:
   ...

editor:
   ...

"buttons.":
  "link.":
    "page.":
      "properties.":
        "class.":
          default: 'somecssclass'
    "properties.":
      "class.":
        allowedClasses: 'somecssclass'

"classesAnchor.":
  "linkIconPage.":
    "class": "somecssclass" 
    "type": "page" 

"classes.":
  "somecssclass.":
    "name": "Some CSS class label"