Is it possible to inject JS before page load, or is it necessary to use content scripts and way for the document to finish?
For example, is there a faster way to execute JS that turns the page red as soon as it's opened?
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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.
