Didn't notice this at first because I was using a straight up tasklist for testing, but after awhile I noticed it was indented too far. This is much more noticeable when you mix list types.

- item 1
    - [X] item A
    - [X] item B
        1. item a

The reason is because the Github Markdown API is not consistent with what actually get used on a github repo or in the comment section.

The CSS is looking for the input item to have a class of task-list-item-checkbox:

.markdown-body .task-list-item-checkbox {
  margin: 0 4px 0.25em -20px;
  vertical-align: middle;
}

But the the API returns all the tasklist items without this class (I don't understand why they don't use the exact same API internally and externally via the API, but they seem to not).

Something like this should be good enough.

.markdown-body .task-list-item input {
  margin: 0 4px 0.25em -20px;
  vertical-align: middle;
}

I will take a look at these within the next week if no one else does. But I wanted to document the issue.