How JS judges different resolutions and calls different CSS style files and test code _ javascript skills

I recently read a website and found that different resolutions of the display and style files are called differently. So I was curious to study and write one. After testing, I felt pretty good, if you are interested, you can take a look at a website recently and find that different resolutions of the display and style files are called differently. Today I wrote an example to study it,

The Code is as follows:

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