STR:
Open data:text/html, <ol start="5"> <ol><li>u</li></ol> <li>x</li> </ol>

Expected result:

   1. u
5. x

Actual result:

   1. u
2. x

Chrome produces the expected result.

Moving Webcompat Priority back to default value.
Feel free to re-nominate with providing a url where users encounter different experience cross browsers due to this defect.

I experience this defect when sending emails in a web interface. When I try to make a nested list in ProtonMail, the numbering is off. I do not experience this problem on Safari. Screenshot attached.

Changing the title because I think the problem is broader than that. As far as I can tell, an <ol> that has another <ol> nested inside will always resume the numbering of the nested list rather than continuing its own numbering.

(In reply to Hsin-Yi Tsai [:hsinyi] from comment #1)

Moving Webcompat Priority back to default value.
Feel free to re-nominate with providing a url where users encounter different experience cross browsers due to this defect.

AFAICT there is no way to see ProtonMail's webmail interface without logging in, so I can't really provide a URL. But I provided screenshots above. If a URL is necessary, here is a JSFiddle containing approximately the same code.