Bug 1004431 - fix broken CSS in OSX's downloads.css, rs=mconley

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extra : rebase_source : 6888ff520f42dbdc84ff622ecdcc2baf246559ce
Bug 971096 - add test to parse all our CSS and check for obvious issues, r=mconley

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extra : rebase_source : 3f17fbda86aa13155c970e157dcec0ae67042696