CSS/JS compression (gzip/Brotli) works only for a short time after cache clearing


I’ve had quite a good search around this question, so apologies if this has already been discussed, but I cannot find anything on it.

We keep an eye on Google PageSpeed Insights (Lighthouse) and recently noticed that we started to be marked down on “Enable text compression” which never used to happen. We’ve not changed any settings in CF or config on our servers which we think will impact this. (Note we do (and always have had) css/js minification enabled in CF.)

We seem to have compression on our HTML (www.viovet.co.uk) but not on our static assets (css/js). Let’s take an example of an asset as https://static1.viovet.co.uk/frontend/scss/Viovet/aggregate.css?1590506055

I’ve noticed that if I clear the CF cache for this file then I start to see the file with content-encoding: gzip response header, see screenshot below. We gzip at our origin server. I don’t understand why CF didn’t compress this further with Brotli. I’d like if it we were taking advantage of Brotli but I can cope with the gzip being passed through.

If I then wait a short while and refresh the file again, I no longer have a content-encoding header at all.

It seems that when the CF cache is hit there’s no compression. There’s also a difference in the transfer size so it looks like it’s not just a missing header, it’s really just no longer compressed.

I’ve tried disabling the Brotli compression option in CF but that doesn’t have an impact as far as I can see, so I’ve re-enabled it.

My understanding is that Lighthouse uses the content-encoding header as the sole source of determining if a file is compressed and then compares that to what it thinks it achievable with gzip (not Brotli).

So, my question is how can I ensure my assets are compressed to the best level possible with the connecting client?

Thanks for your help,


Your origin server is sending cache-control header with no-transform so you’re telling CF proxy not to do anything Cache-Control - HTTP | MDN


No transformations or conversions should be made to the resource. The Content-Encoding , Content-Range , Content-Type headers must not be modified by a proxy. A non-transparent proxy or browser feature such as Google’s Light Mode might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link. The no-transform directive disallows this.

Thanks for the reply. That’s really helpful.

I’ll remove that part of the header now and check that resolves the issue.

Out of interest, do you know why CF is dropping our gzip then as presumably the no-transform directive should be preventing that from happening?

Might be due to enabling Brotli on CF ? As CF natively only supports gzip from origin so AFAIK for brotli at CF edge, it will fetch uncompressed asset from origin and do conversion to brotli. But your no-transform is telling CF edge server not to do conversion to brotli so served the uncompressed asset as is. You can confirm by trying to remove no-transform header. Be sure to purge your CF caches as well.

Thanks @eva2000that makes total sense, and yes removing the header directive has resolved this.

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Glad to hear :sunglasses:

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