Fix some typos, trim passive voice, tighten it up

Yes. the line break after the MD url citation.
The guides have a few instances of the formatting "bug" as shown in the attached image. 
I see why it happens, but it looks sloppy and degrades the professionalism that has otherwise been put into this content.

In the attached image, there is clearly enough line length (purple arrow) to fit the string "Shadow DOM", which ended up on its own line.

This doesn't hurt the understanding of the reader; it just kind of says "amateur".

![selection_049](https://cloud.githubusercontent.com/assets/1242423/4647845/0398f3c6-547e-11e4-9cc1-f7c719b5195e.png)