Consider ways in which we could test user agent's resource blocking behaviors.

https://github.com/whatwg/fetch/pull/1840 aims to explain user agent interventions that block requests for certain resource types and/or replace potentially harmful responses with something benign. While the exact set of resources against which user agents intervene is not (and likely cannot be) standardized, https://explainers-by-googlers.github.io/script-blocking/#testing suggests one approach to testing which might allow vendors to verify that their interventions are consistently positioned within Fetch, but that infrastructure hasn't yet been built or agreed-upon. In short: we could define web driver hooks to allow specific domains or resources to be blocked by the UA's infrastructure, which should give us the ability to verify the web-facing behavior.

Shims/replacements could likewise be tested if similar hooks were created.

WDYT?

cc @annevk @MayyaSunil

Start by reading the Fetch pull request #1840 and the script-blocking testing explainer linked in the issue. Define the proposed WebDriver hooks for UA resource blocking and shims, then establish how tests would verify the resulting web-facing behavior. Done means the approach and required infrastructure are agreed upon.