The standards-track `shadowrootmode` attribute, which enables declarative Shadow DOM, was shipped in Chrome 111 [1]. The older, non-standard `shadowroot` attribute is now deprecated. During the deprecation period, both attributes are functional, however the `shadowroot` attribute does not enable the new streaming behavior, whereas `shadowrootmode` allows streaming of content. There is a straightforward migration path: replace `shadowroot` with `shadowrootmode`. The old `shadowroot` attribute is deprecated as of Chrome M112, and it will be removed (no longer supported) in Chrome M119, which goes to Stable on November 1, 2023. [1] https://chromestatus.com/feature/5161240576393216
The new attribute, `shadowrootmode`, is standards-track and will be supported by other browsers. The older `shadowroot` attribute will not. We'd like to migrate existing users to the new attribute, and eventually remove support for `shadowroot`.