[maaaats]

How is a webpage able to query the local file system? That sounds pretty bad.

[kiallmacinnes]

It doesn't, it queries the local assets of installed extensions. Chrome (and I guess other browsers?) provide a way to do this, so the HTML etc injected by an extension can reference assets shipped with the extension.

[akerro]

You can query static asset of extension by it's path chrome://extensionid/asset.css