Parsoid/How to triage bugs

Right now this is a rough draft. We're trying to collect a set of questions, with detailed guides, on how to effectively triage a Parsoid task.

Is this a VisualEditor problem or a Parsoid problem?
It's useful to use the REST API to view the page to determine whether Parsoid's output is at fault, or if VisualEditor is rendering that output incorrectly. For example:

https://en.wikipedia.org/api/rest_v1/page/html/Main_Page

Change  to the base URL of the wiki you're looking at, and replace   with the page displaying the bug you are trying to triage. Note that you have to convert any slashes appearing in the page title to, for example:

https://en.wikipedia.org/api/rest_v1/page/html/User:cscott%2FIdeas

If the wikitext has been changed since the bug was reported, you can render an older revision of the page by adding the revision id after a slash, for example:

https://en.wikipedia.org/api/rest_v1/page/html/Barack_Obama/97799263

If you are looking at old versions of the page using the History tab, clicking on the date string in the history list will bring you to a page ending in  and then some number. That number is the revision id.

More details on the REST API can be found at https://en.wikipedia.org/api/rest_v1/ -- look for the appropriate section under "Page Content".

WRITE ME: Can also use  to get parsoid output; see T239305#5850245