In a current debug build of MiniBrowser, I hit this assertion if I go to xbox.com/play and click on Sign In.  With logging enabled:

GameControllerGamepadProvider explicitly starting GameController framework monitoring
HIDGamepadProvider opening/scheduling HID manager
GameControllerGamepadProvider has no initial GCControllers attached
GameController framework will handle newly attached device 0x13c665fc0 - HIDGamepadProvider ignoring it
GameController framework does not handle attached device 0x13c75cbe0 - HIDGamepadProvider will handle it
HIDGamepadProvider device 0x13c75cbe0 added
Creating GenericHIDGamepad 0x114004580
MultiGamepadProvider adding new platform gamepad to index 0 from a HID source
UIGamepadProvider::platformGamepadConnected - Index 0 attached (visibility: 0)
WebGamepadProvider::gamepadConnected - Gamepad index 0 attached (visibility: 0)
Connecting WebGamepad 0
WebGamepadProvider::gamepadActivity - 1 gamepad datas with 0 local web gamepads
ASSERTION FAILED: m_gamepads.size() == gamepadDatas.size()
/Volumes/z/safari/main/OpenSource/Source/WebKit/WebProcess/Gamepad/WebGamepadProvider.cpp(106) : void WebKit::WebGamepadProvider::gamepadActivity(const Vector<WebKit::GamepadData> &, WebCore::EventMakesGamepadsVisible)
1   0x1305f2680 WTFCrash
2   0x11aaafe44 WTFCrashWithInfo(int, char const*, char const*, int)
3   0x11cf4ac24 WebKit::WebGamepadProvider::gamepadActivity(WTF::Vector<WebKit::GamepadData, 0ul, WTF::CrashOnOverflow, 16ul, WTF::FastMalloc> const&, WebCore::EventMakesGamepadsVisible)
4   0x11d4f80d8 WebKit::WebPage::gamepadActivity(WTF::Vector<WebKit::GamepadData, 0ul, WTF::CrashOnOverflow, 16ul, WTF::FastMalloc> const&, WebCore::EventMakesGamepadsVisible)
...