shadowrootmode attribute does not work in Blazor files

[ostomachion]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Adding <template shadowrootmode="open"> to a Blazor file produces a regular template element with a DocumentFragment child node.

Expected Behavior

The <template shadowrootmode="open"> should be parsed as a declarative shadow DOM as it would be in a regular HTML file.

Steps To Reproduce

1. Create a new Blazor project in .NET 8
2. Add <template shadowrootmode="open">Test</template> to a .razor page.
3. Run the app in a browser.

Exceptions (if any)

No response

.NET Version

8.0.202

Anything else?

(This rule)[https://developer.chrome.com/docs/css-ui/declarative-shadow-dom#parser-only] seems to be the cause of the bug. When Blazor builds the component, it should account for shadowrootmode attributes and create a shadow DOM if necessary.

NOTE: This was originally reported at #44986, however this was before the shadowrootmode attribute was standardized and widely supported. I am reopening this as a bug report now that this has changed.

Also note that #44986 was marked as a duplicate of #34641 which is requests a very different feature from this bug report.