Update free-courses-en.md (#3848)

* Update free-courses-en.md

Added a few courses under HTML & Css

* Update free-courses-en.md

Corrected indentation error pointed out by Travis, corrected alphabetical order

free-courses-en.md

@@ -212,6 +212,8 @@
 * [HTML Tutorial](https://www.youtube.com/playlist?list=PL_RGaFnxSHWr_FB-hVEgEGUESZL1TOiJ6)
 * [Learn how to program: CSS](https://www.learnhowtoprogram.com/css) - Epicodus Inc.
 * [Learn HTML5 Programming From Scratch](https://www.udemy.com/learn-html5-programming-from-scratch/)
+* [Learn to style HTML using CSS](https://developer.mozilla.org/en-US/docs/Learn/CSS)
+* [Structuring the web with HTML](https://developer.mozilla.org/en-US/docs/Learn/HTML)
 
 
 ### iOS