You know what would be cool, if you could use unix cli tools in the browser. Press control+ , a small console box shows up and run: |grep ...|awk ... And the text of the document gets processed. But if you want to mess with the dom you could: Html.Body | sed ... | jq ...
The "configuration" is just more code that gets compiled in on loading. It's a complete first class citizen unlike extensions for most software which is sandboxed in a separate language and can only see certain APIs.
In Nyxt, StumpWM, Emacs, and other Lisp software you can literally put bugfixes in your config, redefine existing functions, and generally do whatever the hell you want.