Because Riven scales in bonus attack damage in every single ability, attack damage marks/quints is alot more effective than armor penetration. The shield also benefits with bonus attack damage, and not with armor pene. HP quint gives 26 health, which is pretty much early game, but this can be swapped for more damage if prefered.

I use flat magic resist glyphs because i prefer the strong early game, these can be swapped to magic resist/per lvl because mostly, the top champs only have physical damage.

As with all bruiser I use 21 offense and 9 defense.

Offense:
I do not skill the "Lethality" which gives +10% critical strike damage, because with my build I do not have any critical strike chance at all. Instead, I use that point to skill 1 out of 3 in Havoc, this gives me more damage not only early, but throughout the whole game.

Defense:
9 points in defense, priority in armor because most of the champions picked for solo top role is all physical damage. The 1 point in Resistance can also be moved to Summoner's resolve, but only if you use HEAL as summoner spell, instead of flash.

Flash is very good for escapes and engages, it's good for top because the jungler will often gank top lane, because it's the easiest lane to do a successful gank. You can use Heal as summoner spell instead of flash, but remember that this makes you a target for their jungler, and they will probably babysit top lane because of you not having a very good escape. I think heal works pretty good because Riven do have alot of mobility, mostly with her Broken Wings'. Mostly, your enemy will take ignite against you, because ignite is actually more effective than exhaust, making your heal not very effective, if you do use heal, and your enemy have ignite, be sure to use your heal at proper timing.

I use ignite, because as I mentioned earlier, it's ALOT more effective than having exhaust, especially if the enemy champion is a high lifesteal sustained one, such as Irelia and Vladimir.

I've seen alot of diffirent skill sequences, but this is what I use and I will also explain why.

I max Broken Wings', because at level 5, it is your primary damage output if you do the Q>basic>Q>Basic>Q>Basic combo properly. I take both Ki Burst and Valor early, so I have all abilities at lvl 3, this is very important. Alot of people seem to be maxing up their Ki Burst after Broken Wings'. I see no point in this, because your Q already has ALOT of damage. And when you level up your Ki Burst, it does not stun for a longer duration per level, it stays at 0.75 seconds at all levels, meaning the only reason to level it up is because of getting more damage, which is not needed with this build at all.

I max the skills so that I always have 2 points more in Valor, because Riven isn't a true tank, you need the shield, not only when harassing at lane, or getting ganked. But it's also a must in team fights if you want to survive for longer duration. You can also level them parallel, in the way of E>W>E>W>E>W, keeping the E same or one level higher than W. This depends on situations or what you prefer.

Maxing Q is best for snowballing a lane, which is very effective, but otherwise, i recommend to max the E ability

I open up with boots and 3 health potions, because of the mobility, Riven is VERY hard to gank if she have high hitpoints and her Broken Wings' are ready for use. The boots helps escaping ganks, but also i think it helps last hitting a little bit in a way, and also harassing can be effective with movement. But this is my opinion.

I later take 2 dorans blades, mostly because of the hp bonus and the damage, but also because of when combined with a vampiric sceptre, it gives alot of sustain.

If owning, rush bloodthirster, if you need armor, get wriggles. The wriggles should be swapped for a bloodthirster when the rest of the build is completed.

Frozen mallet is recommended over warmogs, because of the CC and the attack damage combined with the HP bonuses.

Maw Of Malmortius is a great item because of the extremely high AD bonuses when at low health, and the shield is great if the enemy team have an owning Ap nuker.

Guardian Angel is almost a must in my opinion, it helps you from getting focused down, it also keeps you alive when you actually are getting focused. And if you die, you can probably be saved by your team mates and escape.

I use the sunfire cape, because of the armor bonuses, but also because of the passive being very effective in team fights, especially if you are in the middle of it, it also helps you stay alive with the HP bonus. If the defensive stats is not needed, and the enemy team is stacking armor, use last whisperer instead.