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Let's say I make a website with no Breakpoint and use a fixed text height of 12.

Then I visit the website from an Iphone with 1024 pixels wide (iphone 12 mini for example). Will my text be unreadable small on the Iphone?

And if so, how do I prevent this issue since the iphone won't break below 1024 pixels. Without needing to use 16+ text height for both desktops and mobiles.. Or am I missing something?

If you want to use a different font size of smaller screens, then you will need to add a breakpoint. There is no other way to do this in HTML.

Is there a difference between pixels and breakpoint? I mean if the phone device has 1024 pixels wide which is almost as much as a small older/B brand laptop, how is is to be broken..

If you add a breakpoint then you can set a different font-size for different devices.
Otherwise the font-size will be the same for all devices.

One thing to keep in mind is that the pixel size (screen size) is different from the viewport size. The viewport size for an iPhone 12 mini is 375px x 812px. When using breakpoints, you want to choose breakpoints that correspond to the viewport sizes you are targeting. Unfortunately, there are a lot of different phone viewport sizes out there and some have odd sizes (like this one at 375). So you need to try and pick breakpoints that a majority of devices will fall into versus trying to target one specific device.

here are some links to sites with lists of mobile devices and their viewport widths:

https://viewportsizer.com/devices/
https://viewportsizes.com/

You can google more sites that have may have other devices.