Run the check
List the exact missing keys and test each physical position once.
Test nearby keys as a control group and note whether modifiers change the result.
Repeat through another port or wireless connection, then another device.
Read the result
A stable cluster of missing positions has a different pattern from a whole-device disconnect.
Limits and next step
System shortcuts and protected keys may never reach a browser page.
Repeat the check after changing one setting or connection. Keep a record of what changed before deciding whether the device needs service.