Split out from T54061#4110198.
Following an IRC discussion in #mediawiki-parsoid on or about 2018-04-05:
* We'll get rid of the special handling of `:`, but preserve the existing appearance by using CSS. So end users won't notice a change, unless they have custom CSS rules with selectors like `div.poem > span` or `.mw-poem-indented`, which will not work anymore. This will address T123216.
* The compact attribute is going to be killed. (We didn't actually discuss this, but everyone appears to agree that it is useless.) This will address T16910.
* We'll try to solve the T124144 <hr> issue as well. Existing patches for that task will be insufficient to deal with the problem when <pre> is used.
* Once those things are done, @ssastry suggested to follow this plan:
** get feedback from Wikisource
** do visual diffs if required as part of that
** merge the patch https://gerrit.wikimedia.org/r/#/c/328887/
** evaluate if there is anything we want to deprecate via linter (e.g. intra-line whitespace)
** reevaluate whether we want to merge this into core as <lines> as the original proposal stood