CSS selectors all exist within the same global scope.
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Every selector has the potential to have unintended side effects by targeting unwanted elements or clashing with other selectors. More surprisingly, our selectors may even lose out in the global specificity war, ultimately having little or no effect on the page at all. Any time we make a change to a CSS file, we need to carefully consider the global environment in which our styles will sit. No other front end technology requires so much discipline just to keep the code at a minimum level of maintainability.