It used to be possible on Manifest v2 with chrome.tabs.insertCSS to inject CSS with runAt: document_start in the details of the object when injecting from the background page

https://developer.chrome.com/docs/extensions/reference/tabs/#method-insertCSS

This appears to have been removed with Manifest v3's chrome.scripting.insertCSS and there doesn't seem to be a runAt option available anymore for CSS from a background page.

https://developer.chrome.com/docs/extensions/reference/scripting/#method-insertCSS

It now seems to default to document_idle

Is there another way of doing it? Putting the CSS file in the manifest file isn't really an option as it needs to create the CSS in the background page.

The new chrome.scripting API does let content scripts be injected at document_start but not CSS.

Thanks for any help with this.