1722896 - Take aboutNetError.js into the 21st century

Status: NEW

(Firefox :: Security, task)

Description

[:Gijs (out for now; he/him)]

There are a lot of unfortunate things happening in this file

In no particular order, and not limited to:

1. we wait for DOMContentLoaded to set the page up. There's no particular reason we need to do this. The script is loaded as a module at the end of the page, so any elements we need are guaranteed to be present.
2. The parameters we need are repeatedly extracted from the query string - we could just do that once.
3. The file is 1000+ lines of toplevel functions, with the main initPage one somewhere in the middle; they should be organized better
4. The logic for classes, descriptions, etc. is completely higgledy-piggledy, so it's very hard to determine what errors get treated in what way.
5. It uses CSSOM where it should be using classes that get used by CSS, or the hidden property where applicable.
6. We use innerHTML for DTD-based strings, that should be converted to fluent. We should have a central mapping of error codes to error message identifiers, and stick to it. This will likely mean we also need to update the docshell versions of this page & script.
7. Passing arbitrary localized text content from the docshell side to the main page is 😬