Do they have an evaluation? When negotiating equity before an evaluation is set, there is always funny money going on. What do you really bring to the table? What will be your position in the long run? Will you be more of a CTO position or a programer? If you're the only one actually creating the website, creating the technology then without you, it's just a good idea. Whatever percent you choose to negotiate for, be prepared to have a well documented, planned argument.