Why does line-height: 1.5; look normal, but line-height: 1.5px; makes the lines super tight? Does adding px really change the meaning?

Yes, it changes the meaning a lot. line-height: 1.5; is unitless, so it means 1.5 times the font size, but line-height: 1.5px; forces the line height to be only 1.5 pixels, which is tiny and crushes the text. That’s why lessons usually use the unitless version for readable, scalable spacing.

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