On both the latest WebKit Nightly "Version 9.0.3 (10601.4.4, r198327)" and the latest stable Safari 9.0.3, font-size: 0 on a textarea causes the Enter key to be ignored rather than typing a newline like it should: http://output.jsbin.com/lubezo/3/
Chrome has the same problem: https://bugs.chromium.org/p/chromium/issues/detail?id=595932
Updated Chromium issue: https://bugs.chromium.org/p/chromium/issues/detail?id=595943
Safari 15.5 matches Firefox Nightly 102 behavior while in case of Chrome Canary 104, the associated bug as mentioned in Comment 03 has been marked as WON'T FIX. Further, on the Chrome bug, it is mentioned that it can be potential fingerprinting issue because it can used to determine "zoom level", which are persistent across sessions and reboots. Thanks!
I don't see the fingerprinting aspect here - first, this is about zero size which is the same at any zoom level; and second, I'm sure that zoom level is very detectable in other ways. I'm leaving this to forms and editing experts to decide whether to close this. We may be compatible with Chrome, but this seems like a weird behavior.
Sorry—I don't think I ever opened an ticket for it but the potential fingerprinting issue being referred to is explained here: https://github.com/mathquill/mathquill/issues/584#issuecomment-255258007 Basically if you set font-size: 1px on the textarea, typing works at a normal zoom level, but not at a 90% zoom level (at least at the time of writing that issue comment)