How much can you make up in a novel?

Depending on why you need to make this change, you can make up a lot. The main thing is to at least keep a link to reality, so that whatever you make up can be perceived as a theoretical possibility. That makes it believable, and therefore more interesting. It's highly likely that most of your audience wouldn't know enough about website design to realize you're making up anything at all, but you can't assume that would apply to your entire audience. People will put down an otherwise good book because one detail makes it impossible for them to suspend their disbelief and ruins everything, which means you should maintain what realism you can to make what's unreal acceptable.

Sure. A novel is a story that is like reality...but it's your reality. Make things happen as you'd like them to.

You can do exactly as you please. It's a work of fiction!

Ever heard of Day of the Triffids or the Quatermass?

It's called sci-fi.

Or even, just using your own imagination.