UPDATE

Not a hint of any movement on this from Xara.

Cloudflare advises is reverts fred.css?anything to fred.css so this approach is no work pursuing.
The good news is fred.hash.css will work.

'hash' above is a hash of the file contents of fred.css.

Take the current ani.css, unchanged since 2015 in the xara offering.
An MD6 hash (Size 24 / Level 16) returns 0d4e71, so ani.css becomes ani.0d4e71.css.
Say now just the Copyright year was changed from 2015 to 2020 , the file published would be ani.6b9288.css.

The point is ani.css would never be fetched from the server until is hash is different. Such a file could be set for a max-age of a year and be taken as immutable.

The files that change more frequently and are currently server-cached, like roe.js and the settings for the fonts used, are handled the same way; there would just be more hashed versions on the server but they are fairly small and when development is done, purged.

A great read is https://csswizardry.com/2019/03/cach...for-civilians/.

This article, https://css-irl.info/troubleshooting-caching/, is more for Xara to suggest how it could be introduced into its coding environment.

Acorn