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Why you should never use px to set font-size in CSS - Josh Collinsworth blog
This is also a very good reason to avoid viewport units, like vw or vh, when setting font size. Those are also static, and impossible to override by the user. At most, a value like calc(1rem + 1vw) might be acceptable, since that still contains rem as a base. Even then, however, I’d recommend using clamp() or media queries to set minimum and maximum values, as screen sizes often go far beyond what we might expect or test.
To differentiate between the two: 1rem is always equal to the browser’s font size—or, more accurately the font size of the html element. rem stands for “root em,” and the root of a webpage is the <html> tag. So, 1rem = whatever the document font size is. (Which, again, by default, is 16px, but can be overridden by the user.) em, on the other hand, is the font size of the current element.
1px is equal to whatever the browser is treating as a single pixel (even if it’s not literally a pixel on the hardware screen).
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