I'd suggest you make an exp variable which increases by a specific amount each time you kill an enemy, say, via a link that is only accessed once their HP drops to zero. Then you could make it so that once the exp variable hits a certain amount, it automatically redirects to a "level up" page. This page / page's links could increase various stat attributes or whatever leveling up means to you and it could also reduce your EXP back down to zero. To simulate leveling taking longer / requiring more effort, you could have the player forcibly moved from area to area and have the monsters award less exp.
There are also ways to set a specific amount of exp that must be obtained regardless of what you fight, which allows the player more freedom, and there are ways for different / higher levels to award different attributes, but it could potentially become a big headache for you if you try to make it too complex and too close to a real battle RPG. I will say I commend you on attempting it if you do, I intend to test and use methods of this myself.
Really, you can do a -lot- if you're just creative enough.