See the code:
https://github.com/cure53/DOMPurify/blob/main/src/purify.js#L1465
The shadowroot attribute, which triggers "old-style" declarative shadow DOM, gets special treatment. See this issue for context: #584
Recently, the declarative Shadow DOM feature has had renewed interest, and as part of that conversation, the shadowroot attribute was renamed to shadowrootmode. Chrome has shipped this new attribute in M111. Chrome also will continue supporting the old shadowroot attribute for some time, as developers migrate.
I believe DOMPurify should be updated to look for both shadowroot and shadowrootmode.
@securityMB