Deprecation: #62855 - "XHTML cleaning" functionality moved to legacy extension

See forge#62855

Description

XHTML cleaning is not necessary anymore, since modern technology bases completely on HTML5. Therefore the according functionality has been moved to the legacy extension EXT:compatibility6.

The following TypoScript option has been marked for deprecation:

config.xhtml_cleaning
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The following PHP method has been marked for deprecation:

TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController::doXHTML_cleaning()
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Impact

The content output of the TYPO3 frontend is not cleaned anymore unless EXT:compatibility6 is loaded.

Affected installations

Any installation having the TypoScript option config.xhtml_cleaning set will have different behaviour in the frontend rendering.

Migration

For TYPO3 CMS 7, installing EXT:compatibility6 brings back the existing functionality.