馃槃 mmm, weird. Let me investigate...
After spending some hours researching, I found that this error only happens in deno 1.5.1 and upper (in deno 1.5.0 works fine). And seems like this only happens by running the tests, but the library seems to work fine.
A workaround may be disabling the tests in the github action or, at least, force to install Deno 1.5.0 (it can be configured here: https://github.com/postcss/postcss/blob/master/.github/workflows/deno.yml#L16)
I'm going to create an issue in Deno repository.
Bug reported here: https://github.com/denoland/deno/issues/8355
Ok, I've updated the deno conversion to remove the changes introduced in 8.1.7, like CssSyntaxError.default = CssSyntaxError;
There's an open issue (https://github.com/denoland/deno/issues/8355) to fix this problem in Deno, but meanwhile I've removed here:
https://github.com/postcss/postcss-deno/blob/master/to_deno.js#L29
So, once this https://github.com/postcss/postcss/pull/1474 is merged, I think Deno conversion should work again.
Thanks!