Adding `@viewport` directive and relatives

Hello,
I noticed that when adding something like
```css
@-ms-viewport {
    width: device-width;
}
```
to enable responsiveness under Metro IE10 in Snap mode, it got absorbed and sucked up in some Nirvana I don't know about.

I just noticed that they were missing in the block directives, and added them. I also added a few vendor-specific variants that I deemed reasonable (I'm not sure if Gecko dropped vendor prefix already, though).