GitHub satanically messing with Markdown - changes 666 to DCLXVI

Adding a period after 666 makes it an ordered list marker.

GitHub declares CSS that renders ordered list markers using roman numerals:

ol ol,ul ol {
    list-style-type: lower-roman
}

Escape the period with a backslash, and you should see the correct output.

While other answers are good at explaining why you have the problem, they haven't given you an exact example of how to fix that.

And it seems that you've already solved it in an imperfect manner, replacing your text with

- `666`. ha.

There's a common trick to escape the dot after the number to make it look like a normal text (and not an ordered list label):

- 666\. ha. (this will render as you probably want)

This seems to be followed by github/markup issue 991, where on ordered sub-list, decimal numerals automatically turns into roman numerals.

I have found the cause of problem. It is CSS

This is the expected way for nested ordered lists to render in HTML.

This is not expected in HTML. https://jsfiddle.net/tf5jtv8s

We don't make any modifications to the default HTML behavior.

ol ol,ul ol{list-style-type:lower-roman}

I don't know CSS but my understanding is that this is the cause of problem. I can get expected result by disabling CSS. (I am from my mobile so I can't use browser inspector)

As mentioned in "A formal spec for GitHub Flavored Markdown", GitHub markdown spec GFM: GitHub Flavored Markdown Spec is built on top of the CommonMark Spec.

And as Tommi Kaikkonen mentioned in his answer, the ordered list is because of the dot following 666. See GFM Spec section 5.2.

As mentioned in section 6.1, any ASCII punctuation character may be backslash-escaped, to avoid this issue.
That means:

- 666\. ha.

(as explicitly shown in ForNeVeR's answer)

That is why that 666 number is changed to roman numerals in a GitHub README markdown.

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Mike Lippert commented:

the 1st element in that list so it should show as i not dclxvi.
Markdown ordered lists ignore the actual number used and number sequentially, and I haven't seen a way to change that.

However, no: it shows dclxvi, because the generated html code is <ol start="666">, which is consistent with the GFM specs:

If the list item is ordered, then it is also assigned a start number, based on the ordered list marker"

(here, '666' is the ordered list marker)

Mike adds:

@VonC For anyone else here's another useful excerpt from VonC's doc link:

"The start number of an ordered list is determined by the list number of its initial list item. The numbers of subsequent list items are disregarded."

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Also, why is the spacing messed up? I didn't catch that in your answer

You get an ordered list <ol> within an un-ordered list item <li>:

<ul>
  <li>
    <ol start="666">
      <li>ha.</li>
    </ol>
  </li>
</ul>

GitHub CSS rules include:

.markdown-body ol {
    padding-left: 2em;
}

If you put 3em, you would get

instead of