Ubuntu 18.04 gajim-nightly issues
As mentioned in #9823 (closed), the python3-css-parser package is missing on Ubuntu 18.04. It seems to have been introduced in 19.10 eoan. Of course, gajim has no obligation to supply nightly packages that work with all supported versions of Ubuntu.
When an Ubuntu user on any pre-19.10 version has gajim-nightly installed using the nightly repository, apt will however decide to try and remove this package because of the missing dependency when performing updates. This might be confusing to users. As the same repository is used across many different version of Debian and Ubuntu, I would simply suggest to mention on https://gajim.org/download/#install-instructions at the end that python3-css-parser is required and therefore it won't work on older versions of Ubuntu than 19.10. Though of course, another solution where those users can continue to use gajim-nightly would also be nice.
I'm not sure if the website is tracked in a repository, otherwise I could perhaps write a PR to fix the information.