While listing an item using ordered list,<ol> what if I want it to list with i, ii, iii n so on..

Answer 1

Include type beside ol and you can do like this
<ol type="i">
<li>one</li>
<li>two</li>
<li>three</li>
</ol>

Answer 2

It would be better to use CSS as this is a styling issue. It's best to separate your HTML markup from the styles for better maintainability.

Use the list-style-type CSS property.
The declaration would be
list-style-type: lower-roman;

https://www.w3schools.com/cssref/pr_list-style-type.asp