Site editor: remove "default" admin CSS

[ellatrix]

What?

We are loading a TON of css in the site editor that never use. The culprit seems to be the colors handle, which in turn enqueues wp-admin. wp-admin in turn enqueues basically all css of every admin screen there is:

array(
    'dashicons',
    'common',
    'forms',
    'admin-menu',
    'dashboard',
    'list-tables',
    'edit',
    'revisions',
    'media',
    'themes',
    'about',
    'nav-menus',
    'widgets',
    'site-icon',
    'l10n'
)

I don't know why this was ever done, but that ship has sailed for the normal admin. For the site editor however, we should be a lot more picky and explicitly add the dependencies we need to our own stylesheet. That's already the case: as you will see, even after dequeuing colors, the handles that we need such as dashicons, common, forms etc. are still loading.

Why?

One of the thousand needles that degrades site editing loading performance. Note that each extra stylesheet is a blocking request.

How?

Dequeue colors. This is added to all admin pages here:

https://github.com/WordPress/wordpress-develop/blob/de38e8eeecf80a7ca5ee870ec68a59897216b33b/src/wp-admin/admin-header.php#L93

Testing Instructions

Load the site editor and check that the list-tables CSS file is gone for example.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: ellatrix <[email protected]>
Co-authored-by: youknowriad <[email protected]>

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[youknowriad]

What about the "colors" stylesheet itself, don't we need it? Isn't it what allows us to handle user color schemes properly.

[ellatrix]

@youknowriad I don't believe so. It's a generated stylesheet that targets old admin stuff. We set our own CSS variables and use those variables in our stylesheets. I tested the "modern" admin theme, and the colours still work in the site editor with this PR.

[youknowriad]

Smoke test this. I can spot any style issue.

[t-hamano]

It looks like the admin CSS has been removed, causing the mobile layout to not display correctly:

We'll probably need to add some display;none styles to the edit-site package.

Furthermore, we'll also need to take into account the case when JavaScript is disabled in the browser. Here's what it currently looks like:

With this in mind, we may need the admin-menu style anyway.

[tellthemachines]

It looks like the admin CSS has been removed, causing the mobile layout to not display correctly

Just came here to comment on this 😅

I wonder if we could prevent the markup from loading altogether in these screens. For the wider breakpoints the content is just overlaying the admin menu anyway.

[andrewserong]

I've put up a revert PR over in #66540 for now.