Added the HTML, CSS and C Programming Handbooks (#6484)

* Added the HTML, CSS and C Programming Handbooks

* Update free-programming-books-langs.md

* Update free-programming-books-langs.md

Author: vineetsingh0809

Diff for: books/free-programming-books-langs.md

@@ -449,6 +449,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Structures and C](https://www.smashwords.com/books/view/644937) - Prakash Hegade
 * [The Basics of C Programming](https://www.phys.uconn.edu/~rozman/Courses/P2200_13F/downloads/TheBasicsofCProgramming-draft-20131030.pdf) - Marshall Brain (PDF)
 * [The C book](http://publications.gbdirect.co.uk/c_book/) - Mike Banahan, Declan Brady and Mark Doran (PDF, HTML)
+* [The C Programming Language Handbook](https://flaviocopes.com/page/c-handbook/) - Flavio Copes (PDF, EPUB, Kindle) *(email address requested)*
 * [The Craft of Text Editing or A Cookbook for an Emacs](http://www.finseth.com/craft/) - Craig A. Finseth
 * [The Current C Programming Language Standard – ISO/IEC 9899:2018 (C17/C18), Draft](https://web.archive.org/web/20181230041359/http://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf) - Open Standards Org - www.open-std.org (PDF)
 * [The GNU C Programming Tutorial](http://www.crasseux.com/books/ctut.pdf) - Mark Burgess, Ron Hale-Evans (PDF)
@@ -941,6 +942,8 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Resilient Web Design](https://resilientwebdesign.com/#Resilientweb%20design) - Jeremy Keith
 * [RTL Styling 101](https://rtlstyling.com) - Ahmad Shadeed
 * [Scalable and Modular Architecture for CSS](https://web.archive.org/web/20191116073929/http://smacss.com/) - Jonathan Snook
+* [The CSS Handbook](https://flaviocopes.com/page/css-handbook/) - Flavio Copes (PDF, EPUB, Kindle) *(email address requested)*
+* [The HTML Handbook](https://flaviocopes.com/page/html-handbook/) - Flavio Copes (PDF, EPUB, Kindle) *(email address requested)*
 * [Understanding Flexbox: Everything you need to know](https://ohansemmanuel.github.io/uf_download.html) - Ohans Emmanuel
 * [W3.CSS Succinctly](https://www.syncfusion.com/ebooks/w3_css_succinctly) - Joseph D. Booth
 * [Web Audio API](http://chimera.labs.oreilly.com/books/1234000001552) - Boris Smus