Our current workaround:

class PolicyMutatedEvent
{
    protected string $sitemapPageType = '1533906435';

    public function __invoke(\TYPO3\CMS\Core\Security\ContentSecurityPolicy\Event\PolicyMutatedEvent $event): void
    {
        $request = $this->getRequest();
        if (
            !$event->scope->isFrontendSite() ||
            $request === null ||
            $request->getAttribute('frontend.controller')?->getPageArguments()->getPageType() !== $this->sitemapPageType
        ) {
            return;
        }
        $event->getCurrentPolicy()->set(
            Directive::StyleSrcElem,
            SourceKeyword::unsafeInline
        );
    }

    protected function getRequest(): ?ServerRequestInterface
    {
        return $GLOBALS['TYPO3_REQUEST'];
    }
}

Thanks for the feedback, I was able to reproduce the behavior...

The resulting CSP violation looks like this:

{
    "document-uri": "https://ip13.anyhost.it/?type=1533906435",
    "referrer": "",
    "violated-directive": "style-src-elem",
    "effective-directive": "style-src-elem",
    "original-policy": "default-src 'self'; script-src 'self' 'nonce-wdB5XPhMdnxXxfNpAsJGflt5UH0o_5-Z4ijHDwQLoQ-03gInUmWx_w' 'report-sample'; style-src-attr 'unsafe-inline' 'report-sample'; img-src 'self' data: *.ytimg.com *.vimeocdn.com; base-uri 'self'; frame-src 'self' *.youtube-nocookie.com *.youtube.com *.vimeo.com; style-src-elem 'self' 'nonce-wdB5XPhMdnxXxfNpAsJGflt5UH0o_5-Z4ijHDwQLoQ-03gInUmWx_w' 'report-sample'; report-uri https://ip13.anyhost.it/@http-reporting?csp=report&requestTime=1714503719476538",
    "disposition": "enforce",
    "blocked-uri": "inline",
    "line-number": 2,
    "source-file": "https://ip13.anyhost.it/",
    "status-code": 200,
    "script-sample": "body {\n                        font-fami" 
}

script-sample gives an indication of what exactly caused the violation.

Patch set 1 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

The patch at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099 is providing those hashes for the XSLT styles.

The hash value might be applied statically as well (sha256-d0ax6zoVJBeBpy4l3O2FJ6Y1L4SalCWw2x62uoJH15k=) - but I think this might be forgotten, in case the styles would be adjusted (even changing the formatting, whitespaces, etc. would require a new hash to be generated).

Patch set 2 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 3 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

From dupe report #104125 a suggestion:

"Maybe by adding a check if CSP is off in the frontend and only then inserting the xsl?"

which has also been voiced here. The current patch seems to have stalled due to caching / performance implications.

Personally I do also think the stylesheet isn't that important to have, and we could just drop it when frontend CSP feature is active, instead of doing all this dynamic processing...?

Patch set 4 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 5 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 6 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 7 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 8 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 9 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 10 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 11 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 12 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 13 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 14 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 15 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 16 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 17 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 18 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/84099

Patch set 1 for branch 13.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/86824

Patch set 1 for branch 12.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/86825