Critical CSS
21 comments
·May 6, 2025todotask2
When I tested mine, I got the following:
Built on Astro web framework
HTML: 27.52KB uncompressed (6.10KB compressed)
JS: <10KB (compressed)
Critical CSS: 57KB uncompressed (7KB compressed) — tested using this site for performance analysis.
In comparison, many similar sites range from 100KB (uncompressed) to as much as 1MB.
The thing is, I can build clean HTML with no inline CSS or JavaScript. I also added resource hints (not Early Hints, since my Nginx setup doesn't support that out of the box), which slightly improve load times when combined with HTTP/2 and short-interval caching via Nginx. This setup allows me to hit a 100/100 performance score without relying on Critical CSS or inline JavaScript.
If every page adds 7KB, isn’t it wasteful—especially when all you need is a lightweight SPA or, better yet, more edge caching to reduce the carbon footprint? We don’t need to keep transmitting unnecessary data around the world with bloated HTML like Elementor for WordPress.
Why serve users unnecessary bloat? Mobile devices have limited battery life. It's not impossible to achieve lighting fast experience once you move away from shared hosting territory.
kijin
Yeah, it's a neat trick but kinda pointless. In a world with CDNs and HTTP/2, all this does is waste bandwidth in order to look slightly better in artificial benchmarks.
It might improve time to first paint by 10-20ms, but this is a webpage, not a first-person shooter. Besides, subsequent page loads will be slower.
stevenpotts
I searched online for tools to extract the critical css of a website for one of my clients, I couldn't find one that did the job so I did so after using Puppeteer locally and then decided to share the solution I used that let's you specify how long to wait after page load to extract the styles; even found a paid one but requested refund after it didn't work.
Feedback welcome, it's free for now.
al_borland
FYI: While a bit of an edge case, as I don’t know why anyone would do this realistically… If a site without CSS is passed, it throws an error.
jefozabuss
What was the problem with something like https://www.npmjs.com/package/penthouse ?
null
promiseofbeans
Is the code somewhere? This seems like it'd be really useful as a Vite/Astro plugin
cAtte_
yeah, doing this manual copy-paste process every time you change something would count as cruel and unusual punishment
sublinear
Non-starter for all but hobby websites since it's incompatible with any content security policy disallowing inline style tags.
Edit regarding replies to this comment: I'm sure many will get a kick out of your workarounds and they're all worth posting in the spirit of HN, however I am talking about CSPs that disallow shenanigans. Carry on though :^)
efortis
You can allow safe inline CSS with a nonce. For example:
<style nonce="sha256-Ce2SAZQd/zkqF/eKoRIUmEqKy31enl1LPzhnYs3Zb/I=">
html { background: red }
</style>
default-src 'self'; style-src 'sha256-Ce2SAZQd/zkqF/eKoRIUmEqKy31enl1LPzhnYs3Zb/I='
https://github.com/uxtely/js-utils/blob/ad7d9531e108403a4146...
its-summertime
Its completely compatible, if you separate dynamic content until after the critical css is loaded: https://posts.summerti.me/being-unsafe-safely/
yakshaving_jgt
That’s a good point, though can’t this instance be whitelisted with a nonce?
sublinear
You could, but in the real world not every frontend dev has control over the CSP on the server allowing nonces to even be an option.
Even when they do they might be subject to a security audit forbidding it. There's two reasons nonces can suck: first is that nonces may be passed around for 3rd party script usage and that blows a hole in your security policy, and the other is that many implementations to generate nonces are not implemented correctly, so the security team might have less trust in devs.
It really depends on the organization and project. Once you start getting near the security fence you may find it's more trouble than it's worth.
I would try to find less complicated solutions for small details like this. Obvious question might be why your CSS can't be a separate file that is small enough to not cause a performance issue.
jer0me
Kind of funny that the agency that made this has a loader on their site.
Brajeshwar
I’ve been away for quite a while, so just a loud thinking.
With tools such as PostCSS, and servers serving zipped styles across CDN, maintaining a single request to the styles; does it really benefit from breaking up the styles these days?
Also, I’m going to assume, besides the core styles that run a website, anything that is loaded later can come in with its specific styles as part of the HTML/JS.
For the critical CSS thing, we used to kinda do it by hand with some automation more as a toolset to help us decide how much to include (inside the HTML itself - `<styles>`) and then insert the stylesheet. But then, we always found it better to set a Stylesheet Budget and work with it.
n3storm
Wordpress plugins and builders like Divi and Elementor has been inserting all css for every page part or component anywhere in the body for years. I hate it. But, thas this critical css means they have beeing doing it right all along?
optimog
No, having an external file makes it cacheable locally. If every new page loads some of the same css again and again, it's a waste of bandwith. You should already have the stylesheet on your computer by then.
worthless-trash
Why worry about this when companies pakage 10mb of javascript. Is this really where the problem is ?
rado
Not working in Safari. Says ‘done’ but Generated CSS box remains empty
Guptos
[flagged]
Nice one. Would be cool if this also handled responsiveness. The need to dedupe responsive critical styles has made me resort to manually editing all critical stylesheets I've ever made.
I also see that this brings in CSS variable definitions (sorry, ~custom properties~) and things like that. Since critical CSS's size matters so much, it might be worth giving an option to compile all of that down.
> Place your original non-critical CSS <link> tags just before the closing </body> tag
I don't recommend doing this: you still want the CSS downloaded urgently, critical CSS is a façade. Moving to the end of body means the good stuff is discovered late, and thus downloaded late (and will block render if the preloader[0] discovers it).
These days I'd recommend:
[0] https://web.dev/articles/preload-scanner