Just so you know, you can't program in HTML, since nothing gets executed.. ever 
HTML is a mark-up language, not a programming or scripting one.
As for the difficulty of making such a robot... it depends on the complexity you're after I geuss.
The only way to find out is to dive in, get each seperate component working first (ie: simply making the arduino drive the bot, make the arduino retrieve GPS data, that sort of stuff) and when you have it all working on a basic level, and you understand what it does, make a new sketch that incorporates several/all of the functions it has to do.