You might use ~13 diodes in series (.7v drop each) to reduce the battery voltage to a point where it can be input to an arduino analog input pin. There are a number of ways to display the arduino analog pin values in a web page.
But that doesn't actually work because an input pin draws no current until the
protection diode starts coducting at 5.4V or so. Diodes only have a roughly
constant voltage drop when there is a definite constant current flowing, but
its temperature sensitive so you'd need a temp-sensor too.
Voltage divider is simple, reliable, drift-free, cheaper.