Under the clipboard API, it says that Chrome fully supports the clipboard.write() method, and Firefox supports it behind a flag.

However, if I try to write a "text/html" snippet, it gives an error that it is unsupported. So, I'd suggest a new section, or expansion of the comments to include information about which mimetypes are supported.

Furthermore, I am also confused, because Firefox and Chrome have different interfaces for this function, which suggests that 1 or both of them don't fully support this API correctly.
i.e. Firefox requires a DataTransfer object, while Chrome requires an array of ClipboardItem objects. These are incompatible interfaces. (It looks to me like Chrome is correctly following the W3 spec, and Firefox still needs to finish implementing this).