fix `XXX:before`/`XXX:after` style

I'm not sure what's the expected style, but I changed the `**` to be consistent in both Chrome and Firefox.

Tested against https://fullscreen.spec.whatwg.org/.

# Chrome
# Before                              After
<a href="https://cloud.githubusercontent.com/assets/203725/5095271/3f774316-6f1d-11e4-8ed6-aa5d3d637e4c.png"><img src="https://cloud.githubusercontent.com/assets/203725/5095271/3f774316-6f1d-11e4-8ed6-aa5d3d637e4c.png" width="48%"></a>    <a href="https://cloud.githubusercontent.com/assets/203725/5095270/3f580726-6f1d-11e4-80cf-84937b9163f2.png"><img src="https://cloud.githubusercontent.com/assets/203725/5095270/3f580726-6f1d-11e4-80cf-84937b9163f2.png" width="48%"></a>

# Firefox
# Before                              After
<a href="https://cloud.githubusercontent.com/assets/203725/5095272/3f7ac52c-6f1d-11e4-942b-ba7fb05f8063.png"><img src="https://cloud.githubusercontent.com/assets/203725/5095272/3f7ac52c-6f1d-11e4-942b-ba7fb05f8063.png" width="48%"></a>    <a href="https://cloud.githubusercontent.com/assets/203725/5095273/3f7e2da2-6f1d-11e4-8784-dcb93db7a693.png"><img src="https://cloud.githubusercontent.com/assets/203725/5095273/3f7e2da2-6f1d-11e4-8784-dcb93db7a693.png" width="48%"></a>