JavaScript and CSS Check for crawlers

This tool checks for JavaScript support by serving uniquely-tagged CSS and JS assets per session and recording whether each was fetched, whether the script was executed, and how long it took. This data is recorded in a public, aggregated report. Additionally, cryptographic bot-identity verification (Web Bot Auth) helps answer: is this bot really who it says it is, or a scraper using someone else's user-agent string?

For site owners and SEOs, the practical implications can be significant. While it's true that "Google renders JavaScript, so client-side rendering is fine," the crawler / bot landscape is much stupider than just Googlebot and Bingbot. The fetch-versus-execute data tells you which bots see your fully-hydrated page and which see an empty shell, which directly determines whether your content is indexable, citable, or summarizable.

For anyone optimizing for agentic browsers and AI grounding, this test is foundational. Some fetch only raw HTML, some execute JS headlessly, some explicitly request markdown via their Accept headers, some are full Chromium instances. Treating "AI" as one thing is a mistake. Concrete data on which agents execute scripts, which negotiate for markdown, and which present verifiable identity lets you serve the right variant to each, prerender for the ones that won't run your JS, and engineer for the way these systems actually consume your content.