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.
With this build, Sivir has great AttackSpeed, AD, Lifesteal, Dodge, and CritChance (98%, or 90% without runes!) She may lack of Health, but her Lifesteal and Dodge makes up for it.
She is a bit quishy in the early game. Make sure you bought "Vampiric Scepter" to have faster health regen. After you bought "Infinity Edge" (around lvl ~14), she'll be way stronger.
The items' cost are expensive, so max out Ricochet first (If you tend to MIA a lot, max out Boomerang.)
I don't put any items to improve her mana either. If you use your "Spell shield" often during fighting, you'll get 125 mana back each time from blocking a spell. So mana regen is not really a big problem.