Android phones do not have Arial nor Arimo. I tested a simple page with "font-family: Arial, Arimo, serif" and got a serif font.
Second, you are overstating the importance of metric compatibility on the Web. Web pages aren’t built with pixel-perfect character placement (unlike PDFs or some GUI frameworks) - text is flowed by the browser, and the web developer cannot specify nor depend on the precise appearance of it, as the appearance of text may differ depending on the browser engine, underlying OS, system scaling, user zoom, accessibility settings etc.
"Hello" in 10px Arial takes up 29x10 px on a Windows 11 PC (in Paint), but it might take up a little more or little less 32 px on Linux, or macOS, a different Windows program, or a 150% scale display, due to antialiasing/hinting/rendering technique differences. On a webpage, you wouldn’t make a button containing the word "Hello" precisely 29x10 px — the button would have more breathing space anyway, or it might just have a margin/padding instead of width/height set, and the layout will work even with tiny rendering differences.
Also, while a handful of very-badly-designed pages may break if you replace Arial with Helvetica for aesthetic reasons, the bigger crime they simultaneously commit is that they won’t work if you replace Arial with OpenDyslexic for accessibility reasons.
Android phones do not have Arial nor Arimo. I tested a simple page with "font-family: Arial, Arimo, serif" and got a serif font.
That’s may be true for Firefox on Android, which has, as per caniuse.com, a 0.28% global usage share. Chrome for Android, which has a 40.28% usage share (well over 100 times the users as Firefox/Android) shows things without serif.
There’s also a number of uniwidth typefaces which do not get wider in their bold form, for GUI elements which need to stay the same size (a use case which we should not completely dismiss as bad design), but those need to be downloaded as .woff/.woff2 fonts.
I have reproduced your results. It looks like Android made the move to using Arimo for Arial to using Roboto fairly recently; the Arial trick worked with my older 2017/2018 Android smart phone. Unfortunate too; there is no longer any universal font stack which preserves metrics. The only option now to preserve metrics is to have users download a fairly large .woff/.woff2 font.
Large is relative; the 116,472 bytes my font stack needs has five fonts: Roman, bold, italic, sans serif bold, and sans serif bold small caps. Mind you, this only supports English and Spanish, but it does have smart quotes, an em dash, and other typographic characters: “”‘’— etc.
Then again, 116,472 bytes is a drop in the bucket in an era when people by and large consume video content online.
Second, you are overstating the importance of metric compatibility on the Web. Web pages aren’t built with pixel-perfect character placement (unlike PDFs or some GUI frameworks) - text is flowed by the browser, and the web developer cannot specify nor depend on the precise appearance of it, as the appearance of text may differ depending on the browser engine, underlying OS, system scaling, user zoom, accessibility settings etc.
"Hello" in 10px Arial takes up 29x10 px on a Windows 11 PC (in Paint), but it might take up a little more or little less 32 px on Linux, or macOS, a different Windows program, or a 150% scale display, due to antialiasing/hinting/rendering technique differences. On a webpage, you wouldn’t make a button containing the word "Hello" precisely 29x10 px — the button would have more breathing space anyway, or it might just have a margin/padding instead of width/height set, and the layout will work even with tiny rendering differences.
Also, while a handful of very-badly-designed pages may break if you replace Arial with Helvetica for aesthetic reasons, the bigger crime they simultaneously commit is that they won’t work if you replace Arial with OpenDyslexic for accessibility reasons.