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.
This is a decent build on Jax due to his mixed scaling on stabilizes along with the changes to hextech gunblade alloying to heal you off the damage mastery and damage for Rageblade. Along with this is his passive and generally scaling with attack speed items. This also gives him immense mixed damage making it near impossible to itemize against.