This build is good because Kennen needs sustain to be able to lane into tanky or semi agrresive champs. (which is still a big part of the meta in toplane)

In these matchup's you want to be harassing the opponnent with basic attacks without having to worry so much about taking a real trade and about dropping minion aggro. And since most champs cant really solo kill a Kennen you should be fine just trying to wear down your enemy and then deny cs as much as possibly or getting damage on tower. Or simply playing safe and spamming TP+FLASH.

Which is actually the coolest thing about this build: you will get a 3.11 min cooldown on TP and Flash with Lucidity boots + Distortion enchantment + Insight mastery.
(This combo I try to get Asap after Zhonya's)

This leads me to my choice of masteries:

I realise that Thunderlords is OP at the moment but I dont think it's better in the top lane with Kennen. Since i really think you need some more sustain/slow harass rather than burst damage. Atleast against tanks, bruisers and champions that can actually deal damage to you. Also I dont think going 0-18-12 would be very good, and i really like that Insight mastery! Furthermore i kinda Deathfire Touch mastery on Kennen, he has some early AD and playing him as a slowly her***ing champ makes sense to me.
Also Deathfire touch will work on several enemies at the same time, which is good for all Kennens AOE abilities. (Thunderlords does not ofc.)

Lets try and have a look at all the sources of sustain:

Items: Doran's Blade(3% lifesteal) + Cull(+3 health on hit)= ca. 4.5 hp per. basic attack

Masteries: 2% lifesteal and spellvamp + Feast mastery (ca.45 Hp pr. min) +2.0 health regen per 5 second+ Runic armor mastery to top it all off! I Would roughly say thats about =
100+ hp per min. And then add the 4.5 hp per. basic attack you get from items.

That is easily 200 hp per min. which is allot imo.

Well this will be all for now, I might add some more things later. But please comment! :)