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.