By "web page" - if you mean a specific host on the Internet, then yes.
/ip firewall filter
add chain=forward action=accept protocol=tcp dst-port=12345 dst-address=x.x.x.x \
comment="allow tcp 12345 to host x.x.x.x"
add chain=forward action=drop protocol=tcp dst-port=12345
comment="block tcp 12345 to all other hosts on the Internet"
This example allows tcp/12345 when the destination is a particular IP address, but the next rule blocks tcp/12345 unconditionally. Since the rules are processed in order, this will allow this port to host x.x.x.x but not to any other IP address in the entire Internet.
Of course you don't need a block rule explicitly on 12345 and right afterwards - if the default rule is to drop everything on all ports if it hasn't been authorized yet, then of course tcp/12345 would be getting dropped as well.
However - this won't allow a specific web page on this specific server, but block other pages hosted on the same port on the same IP.
i.e. you can't use the firewall to allow:
Only the HTTP proxy feature can do that.
(technically, the Layer7 protocol matching feature can do it, but I personally hate using Layer7 feature in a firewall rule chain, and besides, it doesn't work for SSL-protected sites anyway)