[Chris_Newton]

Did you show your designer friend what happened when someone presses ctrl +? Or uses a different browser or loads their own fonts or etc etc etc?

Why would either of those things be a problem? Attention to detail is attention to detail at any scale.

It’s true that at scales smaller than anyone would normally notice there can be a difference between a clean optical alignment and a “perfect” mathematical one. This is a challenge that folks like font designers and artists working on icons often have to face. If you zoom in dramatically (say 5x or 10x, not 120%), these details would probably look slightly off.

However, at the kind of scale we’re using for examples here, zooming in will only exaggerate careless flaws like having things misaligned by a pixel or using the same border-radius for nested elements where concentricity of the rounded corners was intended. A well designed page will continue to look clean and tidy at larger scales, and it won’t mysteriously break just because someone zoomed or had different font preferences.

[the_hangman]

I'm not sure how familiar you are with the zoom functions on web browsers, but zooming in on many web pages can easily destroy the design. This is especially true when using a lot of media queries in your CSS and when using absolute units to position and size elements in your CSS (e.g. using px's for your base font size instead of em's or %).

[Chris_Newton]

I'm not sure how familiar you are with the zoom functions on web browsers

I’m a professional software and web developer, and I currently build web-based user interfaces for a living.

but zooming in on many web pages can easily destroy the design

Sorry, but that’s simply not true if you have any idea at all what you’re doing. The tools to support designs that follow the original intent but are flexible enough not to break just because someone has different default fonts or zoom levels have been around for many years, they still work as well as they ever did, and they are entirely compatible with the more adaptive/responsive designs we often use today.

(e.g. using px's for your base font size instead of em's or %)

These days, it’s more likely to be the other way around IME.

Too many people still rely on received wisdom from the days when browser zooming didn’t adjust the whole page, as all major browsers now do. The original arguments for avoiding px-based font sizes were about allowing users to configure their preferred size in their browser preferences and have web sites respect it instead of overriding it. Today the default font specified by most sites is larger than it used to be (a good thing, up to a point) and if that doesn’t fit the user’s needs then every modern browser will scale it up when the page is zoomed.

Too many people are also making trendy design decisions under the banner of “mobile first” that result in a poor user experience on desktop/laptop systems (or even, ironically, on tablets). Consequently, we get silly things like specifying 30px thin fonts and main page widths/margins as percentages with no other limits, which probably look awesome on the designer’s chosen development device(s) but unfortunately look terrible and can’t be fixed using the usual browser adjustments on much larger and/or smaller screens.