<ul> example you posted doesn’t have a nested
<ul> (if it did, it would be the same.
Discourse may have done / be doing some correction of mark-up, but AFAIK, it is often the browser that “fixes” it. Some may add a tag where they think one should be, others may remove tags where they think one should not be, they might ignore tags, etc. Testing pages in multiple browsers can reveal “it looks good in x and y browser, but bad in z browser” problems, but IMHO using the W3C validator https://validator.w3.org/ is one of, if not the best ways to ensure consistency.