See Issue #45512
It is recommended for <style> and <link> HTML tags
to not use the “type” attribute anymore.
These references state its recommended practice to omit them:
For this reason, TYPO3 does not add this “type” attribute to the mentioned
HTML elements anymore when rendering HTML.
The attribute type is removed from the HTML tags <style> and <link>
by default for TYPO3 Backend and Frontend output.
All installations of TYPO3 that use <style> or <link> tags are affected.
The probability this has negative impact on the user experience is low, however.
If requested due to very old browser requirements for TYPO3 Frontend,
the type attribute can be added via TypoScript options or Fluid
AssetCollector attributes again.