Update custom CSS validation to allow any CSS except `STYLE` close tags. Previously, some valid CSS would be rejected for containing HTML syntax characters, like this example:
{{{
@property --animate {
syntax: "<custom-ident>"; /* <-- Validation error on `<` */
inherits: true;
initial-value: false;
}
}}}
Developed in https://github.com/WordPress/wordpress-develop/pull/10667.
Follow-up to [61418], [61486].
Props jonsurrell, westonruter, peterwilsoncc, johnbillion, xknown, sabernhardt, dmsnell, soyebsalar01, dlh.
Fixes#64418.
Built from https://develop.svn.wordpress.org/trunk@61527
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