Comment by danShumway | Hacker News Reader

Comment by danShumway | Hacker News Reader

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Two things:

A) there is no way for your OS to know what a millimeter is on the 3rd-party monitor you hooked up. So yes, they are in theory roughly tied to real-world dimensions, but in practice, regardless of what the spec says, a browser "pixel" is just another arbitrary unit of measurement that may or may not map to a physical width. This is especially true if you're mixing monitor resolutions on a multi-monitor desktop setup.

When I swap browser windows between my HDPI monitor and my normal monitor, both the reported resolution of the window and the physical size of a "pixel" change. It doesn't remain fixed to 96 pixels per centimeter.

And when I open Chromium, I get different sizes entirely, because Firefox is using a different scaling factor on my HDPI screen than Chrome is. The spec is great, and you're correct, browser pixels don't map to individual physical pixels on a screen. But no, in the real world it's not a completely independent logical unit. In practice pixel densities and screen sizes will affect the physical size of a browser pixel in some scenarios.

B) the effects of tying your layout and column widths to resolution are negative even if your resolution is defined in real-world logical widths. Even if every single browser pixel was perfectly sized on every device so that it was genuinely 96 pixels per centimeter, it would still be a problem to define all of your column widths and font sizes in centimeters.

People are debating the exact definition of the word "resolution" and it does not matter at all to the point I'm making; this is a useless debate. If people are really offended that I used the word "resolution", then fine, swap it out for some other word like "rulers". I don't care what word you use, it doesn't matter. It is still generally advised on the web not to define your layout based on "rulers". If CSS had an "inch" unit, it would be bad practice to make a static layout that set all of its widths in inches. It would still usually be preferable to set font size and to think about column widths/margins using `em` and `rem` units.

A) monitors do report physical size to the OS!

https://en.wikipedia.org/wiki/Extended_Display_Identificatio... > 21 Horizontal screen size, in centimetres (range 1–255). If vertical screen size is 0, landscape aspect ratio (range 1.00–3.54), datavalue = (AR×100) − 99 (example: 16:9, 79; 4:3, 34.) > 22 Vertical screen size, in centimetres. If horizontal screen size is 0, portrait aspect ratio (range 0.28–0.99), datavalue = (100/AR) − 99 (example: 9:16, 79; 3:4, 34.) If either byte is 0, screen size and aspect ratio are undefined (e.g. projector)

This is not perfect of course, may be defeated by cheap adapters, KVM switches, splitters that send image to 2 monitors, etc... And projectors can't know the projected size. More importantly, people sit far from a projector screen; absolute size is not very meaningful without knowing how far away people sit!

You're right that a window spanning multiple monitors has mixed physical size, but that's an edge case that doesn't invalidate taking physical size into account as a goal. Some OSes do re-scale UI when a window fully moves into a monitor with different scaling.

> A) monitors do report physical size to the OS!

And OSes do something with this? When was the last time you hooked up a 1920x1080 monitor to a computer and it changed the reported resolution from 1920x1080 pixels to something else because of the screen size? You're saying that if I hook a 16 inch 1920x1080 monitor or a 12 inch 1920x1080 monitor to a computer that `screen.width` in a web browser is going to report different numbers to make sure I maintain the correct physical dimensions of a browser pixel?

I'm in a weird position here, because obviously you know what you're talking about, but also very obviously if you hook a 1920x1080 monitor up to a desktop computer and full screen a web page, the CSS breakpoints are not going to act differently depending on the size of the monitor. I can observe that fact in front of me across multiple computers right now. So I'm not sure what's going on.

Maybe we're talking past each other or something? But I do know that in practice my smaller Macbook reports a higher screen resolution than my 19 inch giant Linux monitor. And maybe there are OS-level tools someplace to handle that, but they're not available in CSS anywhere that I can find. What you're saying doesn't pass the real-world test of "is it demonstrable on any of my computers sitting in front of me".

> a window spanning multiple monitors has mixed physical size, but that's an edge case

Is it? If I want my Linux box to scale differently on different monitors, that's something I need to manually set up as a user, at least from most distros I've used. Maybe Ubuntu or Windows does something else? The closest I've ever seen to this is my monitor scaling for different aspect ratios when I mirrored Gnome to a projector or a monitor of a completely different resolution.

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But again, this entire topic ends up being kind of a weird thing to be arguing about because it does not matter if you're using real-world centimeters. It doesn't change anything about what I'm saying, and I almost regret even bringing up "pixels" in general because it seems like they're a giant distraction to everyone.

You should not be building layouts in centimeters. Centimeters will have all of the same problems with scaling fonts as any other fixed unit, the reliability of your measurement doesn't change that. People are fixating on the individual technical details of what's going on in monitor firmware instead of the broader point that in most cases container dimensions should be defined using relative units that scale correctly as browsers/users alter font sizes.

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