How exactly would you block this behavior without getting rid of a substantial portion of web functionality, even around simple document styling?<p>It's not a Javascript problem. To make it impossible, we'd need to get rid of invisible spans. Text overflow can't be hidden. This means you can't display extra text to screen readers, since that's invisible text. Also, non-system fonts are right out, because they can contain invisible characters, or even be remapped so that the wrong characters display.<p>The 'solutions' I'm seeing proposed on this issue are hacks. If this is a real problem, the real answer is to just make the clipboard visible when you copy, preferably on an OS level (since literally every format/platform that allows bundling custom fonts is vulnerable to this, including PDFs).<p>Prefer security solutions that are simple and universally understandable, rather than solutions that rely on adding a bunch of code to plug part of a hole. Doing real-time analysis to figure out whether text is visible doesn't fix the whole problem, and is highly error prone.<p>I think Mozilla is right to reject this. If you're coming up with hacks about per-domain character recognition that will end up behind some kind of permission prompt that users will click through without reading anyway... that's a sign you haven't thought hard enough about what the problem is. When something is written to the clipboard, just bring up a notification on-screen and show the user what they copied, and give them the option to inspect/edit it in more detail. The best thing is that's an OS-level mitigation, and not another weird, buggy implementation detail that makes it harder to build or inspect a web browser.