What happens when we use ol tag as opening and ul tag as closing. Its interesting happened see answer below.

Answer 1

https://code.sololearn.com/Wbv7Yh716aVu/?ref=app

Answer 2

It happend nothing at all... as browsers are intended to correct malformed/invalid html ^^
But if in this case correction is obviously done, as the error is relatively explicit (wrong closing case), it may be less explicit, and you cannot really predict what will happen in others browsers, and how it will be corrected :P
However, search engine would downrank your page/site if not valid Html (they are not mandatory as browsers to open the Html document as well as they can)...