[RavSinghChandan]

e.g. Fixes #0

Checklist:

• A unit test is covering the code added / modified by this PR

• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• A mention of the change is present in CHANGELOG.md

• This PR is ready to be merged

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[RavSinghChandan]

<li value="5"> was ignored, so an ordered list that sets an item's number, as HTML generators do when a list is split by other content, restarted from the wrong number. Browsers number that item 5 and the following items 6, 7, and so on. write_html() now does the same for ordered lists of any type (<ol type="i"> gives iv, v). An invalid value is ignored, and value on a <ul> item still has no effect, as in browsers. Added tests.