Chrome extension: Inject JS before page load
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?
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.