Render Blocking Resource Checker
Find the scripts and stylesheets that hold up first paint, and which of them genuinely need to.
About this tool
Lists every stylesheet and script in a page's head and says which of them block the first render — and, more usefully, which of those had to.
A stylesheet in the head blocks rendering on purpose: it is what prevents the page flashing up unstyled. Flagging each one as a problem is how these reports turn into noise. The findings that matter are narrower.
A **synchronous script in the head** stops the parser dead until it has downloaded and run, and almost every one of them could carry `defer` instead. A **third-party stylesheet** hands your first paint to a server you do not control — if their CDN is slow this morning, your page is blank this morning. And a **long chain** of blocking requests costs a round trip each, which no amount of file-size optimisation recovers.
This reads the HTML. It does not load the resources or time anything, so it reports what blocks, never how long for.
Common questions
What is the difference between async and defer?
Both download without blocking the parser. An async script runs the moment it arrives, interrupting parsing at an unpredictable point; a deferred one runs in order after parsing finishes. For anything that touches the DOM or depends on another script, defer is the safe choice.
Should stylesheets be deferred too?
Not the ones the visible part of the page needs — deferring those produces a flash of unstyled content, which is a worse experience than a slightly later paint. Stylesheets for below-the-fold or print are worth loading conditionally with a media attribute.
Why does a third-party stylesheet get flagged?
Because it blocks your render on somebody else's uptime. A font or icon stylesheet from a CDN is a single point of failure for your first paint, and hosting a copy removes both the dependency and a DNS lookup.
Does this measure how slow the page is?
No. It reads the markup and reports what blocks. Timing needs a real browser on a real connection — this tells you what to look at, not how bad it is.