What's stopping you from using cm/in as your unit (which is actually also what px is based off of in the web, not physical pixels)? ° doesn't make sense until you pick a viewing distance, at which point you're really back to some scaled value of cm/in.
Fixed size layout runs into problems where you're working with displays that aren't at typical desktop / laptop / mobile distances from the retina.
The two obvious examples which come to mind are mounted displays (from wall art and signage to billboards), and glasses-based displays.
A 1cm font size on a mobile device is ludicrously large. On outdoor signage it's invisible, and quite possibly below pixel size. It might be appropriate on a desktop display for some major title or site branding. It's going to fill the entire field of view on a face-mounted display.
The simple truth is that design has to respect not only device capabilities (your colour-palette likely works poorly on monochrome e-ink devices, ask me how I know), and reader capabilities (colourblind? glaucoma? cateracts? presbyopia?, macular degeneration?) but also the specific use case and environment, all in ways that the author of a site or design often has no possible insight on. Client specifies design is the only option which works in all of these instances, and yes, this means that the more complex your site / SPA (o hai ubrz) the more likely it will be to break irreparably in a large number of instances.
Not gonna rewrite or patch a whole CSS framework to make a dashboard. One can avoid using it in the first place, but then has to cope with elusive y-scroll in line inputs, misalignments and so on. There's always something strange out of box even if you target a specific browser.
Which CSS framework out of curiosity? I don't know many that won't let you use px, which is 1/96th an inch at any DPI, and if it really doesn't let you use the most basic sizing element on the web then the problem seems more with the chosen framework than anything to do with browsers :p.
> cm/in as your unit (which is actually also what px is based off of in the web, not physical pixels)? ° doesn't make sense until
Nope!
A CSS pixel is 96 DPI at 28 inches from the viewer.
px is fundamentally angle-based. 1/47 of a degree. And sure it's a "scaled value of cm" at some point but this way the scaling is more obviously handled on a per-device basis.
