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

[talldan]

Trac ticket: https://core.trac.wordpress.org/ticket/61704 Gutenberg issue: https://github.com/WordPress/gutenberg/issues/63712 Gutenberg PR: https://github.com/WordPress/gutenberg/pull/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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[talldan]

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

[aaronrobertshaw]

Thanks the @since and @ticket comments LGTM 👍

[talldan]

Committed in r58856