Seems this is a general issue on systems without Segoe UI (e.g. macOS)

This could be implemented along with #3588

Hi, I've found a free font similar to Segoe UI from other forums, if you could consider it.
WeblySleek UI http://fontpro.com/weblysleek-ui-font-9838

Same problem occurs on other OS/browsers - e.g. Safari on iOS

any known workarounds? setting a global font via css unfortunately doesn't solve this issue

Yes, this is something we need to address asap... I will be working on the icon font and will try to look into this as well. Microsoft is suggesting https://docs.microsoft.com/en-us/typography/font-list/selawik as the open-source alternative to Segoe UI, so that could be a potentially viable solution

Selawik - Typography

Typographic info for the Selawik font

Thisis also a problem for mobile browsers - for example {ThemeResource TitleTextBlockStyle} renders very bold typeface on Android as compared to desktop browser

I like the "system native" font selection fallbacks for one of the apps I'm working on, but something odd happens on macos+chrome where font-weight is completely ignored, which doesn't seem to happen with any other combination of browsers/platforms/targets as far as I can tell.

When I check chrome dev tools and select an element, it says font-family: "Segoe UI" and shows that it is actually rendering Helvetica Neue, but font-weight does not do anything. If I change font-family directly to Helvetica Neue then the font-weight setting kicks in and starts to work.

I also noticed that if it falls back to sans-serif (i.e. if you remove the Helvetica Neue fallback) then it renders times new roman, which might be related to the initial problem. If I change font-family to sans-serif in chrome dev tools then it shows a suitable font.