Thread:Project:Support desk/User Preference descriptions - Where stored & changed, and where documented? - How to mark the "defaults"?

I suggested at en.wiki, that default preferences should be marked, and I'm bringing the synopsis here.

Ideally, we mark the defaults within mediawiki itself, so that all wikis can benefit. (Versus, us hacking together a custom fix at every language/project).

It would be best if they were all marked within the Special:Preferences page/tabs itself, and automagically updated when a sysop changes the default itself (rather than, any change requiring a separate edit to a documentation page).

A few defaults are mentioned at m:Help:Preferences and mw:Help:Preferences and etc, but not many, and the docs are not always up-to-date.

There are no specific bug-tickets that I can see. However 17188, 18961, and 33327 ask for, or hint at this, in the comments.

Context / rationale:

Marking ALL the Special:Preferences defaults, on every tab, would be helpful. There's the "Restore all default settings" button-of-doom which wipes out all our customizations (except email-address, afaik), but often I'll want to make sure that I'm seeing a page the way most other editors do. Eg. setting thumbnail size defaults. I know I've changed a few options at Special:Preferences, but I'm not sure which...

Options, for the visual indicator of "default":
 * Markers. (Placed either next to the check-box/form-field, or after the description. Needs to support true/false checkboxes, and strings.)
 * At Commons:Special:Preferences there are superscripted d markers for the default options.
 * A coloured-text marker, e.g. " ( default ) ", as suggested by User:קיפודנחש at the en:wp:VPT thread.
 * A mouse-over tooltip. (Downside: Requires user-discovery. Also hard to see at-a-glance how many items I've changed from default.)

Documentation:

I [//www.mediawiki.org/w/index.php?title=Special%3ASearch&profile=all&search=preference+description&fulltext=Search searched] for 30 minutes, but got very lost. I was hoping to find a page that explained where the descriptions are kept. Manual:$wgDefaultUserOptions seems to contain info on changing the values, but not the descriptions. Possibly it's not documented anywhere? (Or possibly I've just had insufficient coffee and/or experience with mediawiki...).

Hope that helps!