Doesn't sound unreasonable to me. By the way, the Polish Wikipedia currently has a default-on gadget that implements styles for the main page (which is as mobile-friendly as it gets, I think, and almost free of inline styling). https://pl.wikipedia.org/wiki/MediaWiki:Gadget-main-page.css

(Somwhat related: T483: RfC: Allow styling in templates.)

+9000

I would like some sandbox mechanism though, that allows me to test CSS wihtout having to load it manually via importStylesheet, withCSS or some other hack.

(A default gadget just loads indiscriminatelyon all pages, just as Common.css, so not a good idea.)

See also T32405 - this would allow us to deprecate a bunch of code debt we have in MobileFrontend.

There might be more than one page, where the main page CSS needs to be loaded:

• Commons has different main pages depending on your language (e.g. Hauptseite for German). Of course, this could be solved the same way as Wikidata did.
• Some projects (well, at least one) have an archive of old main pages (e.g. yesterdays main page of the German Wikipedia)
• Some projects have a preview of tomorrow's main page (e.g. tomorrow's main page of the German Wikipedia
• There are also many pages in the user namespace, which are more or less copies of the main page and may or may not need the CSS (e.g. this copy)

Perhaps this can be solved by use of some keyword or link, making it more universally applicable in the long run, ie. [[css:MediaWiki:Mainpage.css]].

Should I create an RFC for this?

In T78418#1199049, @Jdlrobson wrote:

Should I create an RFC for this?

Yes please.

We already have several extensions that can load page-bound CSS. Why are these not used?

In T78418#1437935, @Edokter wrote:

We already have several extensions that can load page-bound CSS. Why are these not used?

I'm not sure I understand what you mean. What existing extensions are you referring to? This is task is about enabling wikis to author a stylesheet on-wiki that is loaded on the Main Page only.

I'm saying we could use existing code for that, for example: https://www.mediawiki.org/wiki/Extension:CSS and https://www.mediawiki.org/wiki/Extension:NewPageCSS

In T78418#1437616, @Krinkle wrote:

The problem with these pages is that they need hardcoded infrastructure in MediaWiki core, tend to be loaded in the global scope, and have no well-defined behaviour for when and how they are expected to be loaded. Gadgets are more mature in that sense.

This is just a CSS page, so global scope isn't an issue. The behavior for this page seems pretty obvious, it's whenever $title->isMainPage() is true.

At this time gadgets do not yet have the capability to be enqueued in the loader for specific page contexts only, but that's something we can work on. Let's file a task for that instead.

That's T63007: Allow specifying when a gadget should load (action, namespace, content model), which is doable in 2.0.

As an alternative for the interim, one could easily add a hook in wmf-config or WikimediaMessages that will check whether the page isMainPage() and that a module ext.gadget.mainpage exists, and then pass it to addModules() or addModuleStyles().

Let's not add more hacks upon hacks please.

Most of the styling should be done using TemplateStyles. The rest (e.g. styles affecting the page title) can be put in a gadget once T241524 is done (which is not bound to the main page, so the necessary styles can be loaded on localized main pages, yesterday/tomorrow pages, sandboxes and the like as well).