Enable start number attribute for ordered lists
Setting the start number
[start=5] on an ordered list doesn't work. I'm not sure how the UI is ignoring/overriding this so this might actually be an upstream error. The tests for this attribute (upstream and in the UI) should also be reviewed to make sure they're transmitting warning/error messages if the value isn't a number and/or that the value was ignored for some reason.
An example for testing can be found in https://gitlab.com/antora/demo/demo-component-a/blob/master/modules/ROOT/pages/lists/ordered-list.adoc
- Fix/enable setting the start number on an ordered list