brent Yes, ungoogled-chromium is a standalone browser that you download for Solus. It is in fact a fork of Google Chromium, with all the Google parts taken out. This makes the browser more light-weight and privacy respecting. It looks and behaves just like Chromium.
As for "headless mode", this is a way to use the browser without UI components. Essentially to use the browser like a command-line tool. Firefox has a headless mode as well:
Chromium has a HTML to PDF converter built into it, which is used for printing websites. Quite usefully, you can access this HTML to PDF converter through headless mode. Meaning you can convert HTML documents to PDF using Chromium on the command-line. It functions the same as wkhtmltopdf. While wkhtmltopdf uses Qt's WebKit rendering engine, ungoogled-chromium uses Chromium's rendering engine which I believe is called Blink.
Since this feature is part of Chromium, it means that any Chromium-based browser (Vivaldi, Brave browser, Microsoft Edge) can be used in headless mode to perform these HTML to PDF conversions. I choose to use ungoogled-chromium for the afore mentioned reasons (light-weight and privacy respecting).
Description of the flags used in my previous comment:
--headless is used to enter headless mode in Chromium (use Chromium as a command-line tool)
--disable-gpu is optional, since there's no UI components we can disable GPU. If not used, you'll get a warning message about GPU, but the command will still work
--print-to-pdf is used to access the HTML to PDF converter in Chromium.
Hope this helps