Display the numbers in edit mode

Numbering using CSS in edit mode is easy until you need to restart numbering from an arbitrary value.
Using css, several solutions can be considered:

• take into account a start attribute on the hX tags. This requires to generate a specific rule for each possible value of the start attribute (all integers is a lot), so a lot of css (which can be generated using less) but even with less, resetting to value above 100 is difficult.
• introduce a "style="--start-value=''XXX"" attribute on hX. This works fine but makes the serialization less readable (which is fine as long as one does not switch to xwiki edit mode). This solution is 100% css and quite straightforward
• introduce a listener monitoring heading create/update and generates dedicate CSS rules on the fly