Adding Code Guide to HTML/CSS section.

free-programming-books.md

@@ -856,6 +856,7 @@
 ###HTML / CSS
 * [A beginner's guide to HTML&CSS](http://learn.shayhowe.com/html-css/)
 * [An advanced guide to HTML&CSS](http://learn.shayhowe.com/advanced-html-css/)
+* [Code Guide: Standards for developing flexible, durable, and sustainable HTML and CSS](http://mdo.github.io/code-guide/) - Mark Otto
 * [Dive Into HTML5](http://diveintohtml5.info/) ([PDF](http://mislav.uniqpath.com/2011/10/dive-into-html5/)) - Mark Pilgrim
 * [GA Dash](http://dash.generalassemb.ly)
 * [Google's HTML/CSS Style Guide](http://google-styleguide.googlecode.com/svn/trunk/htmlcssguide.xml)