This is a fork from the WebDFU demo (WebUSB DFU), tailored for flashing OWL devices using the legacy DFU bootloader.
WebUSB is currently only supported by Chromium / Google Chrome.
For Chrome to communicate with a USB device, it must have permission to access the device and the operating system must be able to load a generic driver that libusb can talk to.
On Windows, that means that an appropriate WinUSB/libusb driver must first be installed. This can be done manually with programs such as Zadig or automatically (sometimes...) with WCID
On Linux, that means that the current user must have permission to access the device.
On macOS, it should work directly.