TYPO3 Core

Currently any value given in the ViewHelper would be output exactly like it was given.

<f:asset.script async="1"...

will produce

<script async="1" ...

As these are boolean HTML attributes they are only allowed to be empty [HTML5] or contain exactly the tag attribute's name (async="async") [any].

As such it would make sense to change the ViewHelpers' parameters to boolean, too.

In order to not break anything and keep the output in line with PageRenderer's page.includeJS it should output

<script async="async" ...

References:

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link