Merge pull request h5bp#56 from aharris/patch-1

Typo unncessary = unnecessary

posts/box-shadow.md

@@ -6,4 +6,4 @@ polyfillurls:
 
 `box-shadow` is safe to use most of the time, except when you are applying it on a large area as that may cause significant performance impact when interacting with the page (on scroll and hovers, but also transitions and animations). 
 
-Polyfills are unncessary for box-shadow. They are too costly and slow down pageload. Users without a modern browser will not see a shadow, but aren't checking your site in multiple browsers anyhow.
+Polyfills are unnecessary for box-shadow. They are too costly and slow down pageload. Users without a modern browser will not see a shadow, but aren't checking your site in multiple browsers anyhow.