Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
I don't know English well so I only can explain something about items and runes.
I thing ap runes's good for start game give more dmg from Null Sphere and HP runes's good beacuse Kassadin haven't so much HP at start. Mejai will be first item because when you faster get stacks you have big advantage on 1x lvl with this. Shoes for armor penetration. I think guinsoo is good because make attack faster and give more AP at gank. Rest of items i don't need to explain why :P