Update HiDPI CSS `@media` rule

Provide greater cross-browser support. Thanks to Wordpress -
http://core.trac.wordpress.org/changeset/22629 - and @marcedwards - see
issue #1127.
Adjust CSS image-replacement

Make sure that any text-highlight, or other factors that may break the
illusion, are far enough outside the element to be unnoticed.

Fix gh-1239
Remove named function expression in plugins.js

Details on various bugs with named function expressions:
http://kangax.github.com/nfe/#jscript-bugs
Minor amend, closing a missing bracket.