Chrome extension: Inject JS before page load

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Declare a content script in the manifest file with "run_at": "document_start" to get it to run as soon as possible, i.e. right after constructing the document root (when <head> does not exist yet).

For your very specific example, it might be better to declare a content style instead, similar to content scripts, but using the "css" key instead of "js".

If you want to dynamically run a script as soon as possible, then call chrome.tabs.executeScript when the chrome.webNavigation.onCommitted event is triggered.

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