GitHub has improved their code reviewing tools:
Effective code review catches bugs before they’re deployed, improves code consistency, and helps educate new developers. We’re adding new features to make code review on GitHub faster and more flexible.
A few include a handy timeline indicator showing what you might’ve missed after reviewing a pull request, and a way to filter filters in a pull request by type (e.g. maybe you just want to look at the CSS files).
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More code review tools (on GitHub) is a post from CSS-Tricks