Add section on CSS selector side effects when using Activity mode

[PriyaV2307]

Adds clarification about CSS selector side effects when using .

CSS selectors such as :has(), :is(), and :where() may still match elements that remain in the DOM even when hidden. This can lead to unexpected styling behavior.

This update extends the existing documentation about DOM side effects and helps developers better understand this behavior.

Closes #8354

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[github-actions]

Size changes

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