Ensure root selector (body) is not wrapped in :root :where()

[talldan]

Trac ticket: https://core.trac.wordpress.org/ticket/61704
Gutenberg issue: WordPress/gutenberg#63712
Gutenberg PR: WordPress/gutenberg#63726

What?

In WordPress 6.6, there was a general effort to lower and level specificity, but a couple of selectors seem to have mistakenly had their specificity raised.

The styles associated with the body are an example. In 6.5 these styles used the simple body selector. In 6.6 the selector is :root :where(body), an increase in specificity.

Why?

These changes may cause some problems for themes, especially the typography styles in the body, which will cascade.

How?

Reverts the selector back to body

Note that unlike the gutenberg PR, the core backport only affects the frontend styles. The editor styles will be fixed by updating the packages from gutenberg.

props to @aaronrobertshaw for pointing me to the fix

Testing Instructions

1. Open the site editor
2. Click the editor area to open the editor
3. Click 'View site' to view the frontend
4. Insect the body element's styles
5. Using the browser dev tools, check that the styles are like 'expected' snippet below, with a simple body selector and not a :root :where(body) selector.

Expected:

body {
    --wp--style--root--padding-top: 115px;
    --wp--style--root--padding-right: var(--wp--preset--spacing--50);
    --wp--style--root--padding-bottom: 115px;
    --wp--style--root--padding-left: var(--wp--preset--spacing--50);
    color: var(--wp--preset--color--contrast);
    background-color: var(--wp--preset--color--base);
    font-family: var(--wp--preset--font-family--body);
    font-size: var(--wp--preset--font-size--medium);
    font-style: normal;
    font-weight: 400;
    line-height: 1.55;
}

Before:

:root :where(body) {
    --wp--style--root--padding-top: 115px;
    --wp--style--root--padding-right: var(--wp--preset--spacing--50);
    --wp--style--root--padding-bottom: 115px;
    --wp--style--root--padding-left: var(--wp--preset--spacing--50);
    color: var(--wp--preset--color--contrast);
    background-color: var(--wp--preset--color--base);
    font-family: var(--wp--preset--font-family--body);
    font-size: var(--wp--preset--font-size--medium);
    font-style: normal;
    font-weight: 400;
    line-height: 1.55;
}

---

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Props talldanwp, andrewserong, aaronrobertshaw, mukesh27, hellofromtonya.

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[andrewserong]

This is testing nicely for me, with the styles for the root selector now being applied to body instead of :root :where(body) 👍. Tested with a couple of different block and classic themes.

The code change also looks good and simple. I think this will be a good fix to get in for 6.6.1. LGTM!

[aaronrobertshaw]

Thanks @talldan 👍

This is testing as per WordPress/gutenberg#63726.

Before	After

Before we get this comitted, I think we will need to add a @since 6.6.1 comment as well as @ticket 61704 comments to all the affected tests.

[talldan]

Thanks for spotting that. Linty things should be all fixed in the last two commits 👍

[aaronrobertshaw]

Thanks the @since and @ticket comments LGTM 👍

[hellofromtonya]

These 2 @since shouldn't be necessary, as they are both bugfixes. @since is added for changes in functionality, function parameter(s), return types, filters, etc.

The commit history will provide the context of the bugfix changes.

Suggested change

	* @since 6.6.1 Avoid applying `:root :where()` wrapper to top-level element-only selectors.
	* @since 6.6.2 Avoid applying `:root :where()` wrapper to root selectors.

[hellofromtonya]

• Tested before and after on trunk and the 6.6 branch (see Test Report) - can reproduce and this patch resolves it ✅

• Double checked changes with its Gutenberg PR - confirmed its a clean merge of PHP changes ✅

LGTM for commit ✅

[talldan]

Committed in r58856