If that's a horribly broken assumption on Safari, I'd be grateful if you could explain to me how it's broken.
That would be like if every television programme broadcast audio at a different volume, so every time you watched something new you had to adjust the volume. It’s insane. The browser already knows what size text the user prefers. The only thing you have to do is let them use it instead of overriding it with absolute units. If you’ve designed something that the user needs to change their settings to read, you’ve made a really bad design.
> all have to be in ems if you want things to scale proportionally when font size changes.
You don’t want most things to scale proportionally when font size changes. The goal of a good web design is not to have the exact same raster image appear at different sizes, the goal of a good web design is to be the best presentation for the current situation – which includes user preferences, what content you are displaying, the size of the viewport, etc.
So, for instance, if you have a main content area in the middle of the page and sidebars on each side, then you might want to set a minimum and maximum width for them in ems so that the line lengths don’t get ridiculously long and it doesn’t get squashed, but the width should probably be proportional to the available horizontal width, so percentages or fr should be used for that property.
There are some things that are closely related to the font size, so for instance you should use ems to set padding around text and margins between paragraphs. But that doesn’t mean that, say, the border should be set in ems.
Basically, use the units for their designed purpose instead of thinking you need to use all of the same types of units everywhere.
Well. No. It would be more like if every television program was displayed on a screen that had a different height, width, and scaling that may depend on the state of the browser at any given moment. Which is in fact the case. And is kind of the point of what we're trying to deal with here.
What I do not get for the life of me is why you would introduce a dependency on a deeply obscure user preference that I, as a perfectly literate computer user, would not expect. As opposed to scale, which is completely in your face (and also has an obscure default setting in exactly the same place if that's what the issue is for you).
And it's not like there aren't enough dependent variables I have to deal with to make sure that my UI is responsive across all common page sizes my app will encounter without introducing another one, whose benefit, frankly, just doesn't seem that compelling.
Asking because I genuinely do not get it.
The only thing you have to do here is not do anything. Literally just don’t use fixed font sizes. It takes extra effort to make life worse for people with vision problems. Just… don’t do that. It’s easier than fucking it up.
Why? I’ve started to use rlh as my base unit for almost everything, and it’s great as it automatically gives your whole design consistent vertical rhythm.