Describe when native CSS4 ':has()' can be used and how it is handled in browsers without native ':has()' support

[Alex-302]

• Add examples for CSS4 ':has()';
• Describe limitations of CSS4 ':has()' or add the link https://www.w3.org/TR/selectors-4/#relational and which rules can be converted to native;
• Describe how CSS4 ':has()' handled in browsers without it's support (old Safari, Firefox without experimental flags), including cases where the selector is not supported by CSS4 (:has(:contains(aaa)));
• in which cases native implementation may fail to hide an element;
• performance comparison.