Will browsers immediately fail the connection, or will it try to make the connection and wait for a timeout? I generally use 0.42.42.42 because Class-A addresses can't start with 0, so they fail immediately.

If you're running a web server locally on port 80, the webserver will handle the request. If no service is listening on port 80 your networking stack will not be able to open a tcp connection and it will be actively refused, unless you're using some kind of terrible software firewall.

Using Firefox in Windows 7, it takes 4-5 seconds to fail. The other address I gave fails so fast sometimes the favicon doesn't even blink.

IIRC, addresses 0/8 addresses are for multicast, so depending on your and your upstream's router configuration, that potentially sends those requests to a weird set of hosts on your class-A.

Multicast is 224.0.0.0/4 (224-239). 0/8 should only be used as a source address for "local network" traffic and never a destination.

This would depend on your network setup - it seems unlikely that the OS would automatically refuse to route a request to there. On OS X:

    $ curl --connect-timeout 5 -v 192.0.2.1
    * About to connect() to 192.0.2.1 port 80 (#0)
    *   Trying 192.0.2.1...
    * Connection timed out after 5011 milliseconds
    * Closing connection 0
    curl: (28) Connection timed out after 5011 milliseconds

So your browser will likely end up trying the connection and timing out after a while.

it does wait for a timeout, so "route" was poorly chosen. it just wont get anywhere on most configurations because they respect the 192.0.2.x is examples only convention.