In the course of working with Site Rubix and WordPress, I came to realize that I needed to get better acquainted with HTML.  I've come across a free online site that seems pretty decent so far--

http://www.w3schools.com/html/html_getstarted.asp

It's got it's own practice editor so you can experiment as you like when you like.  It's broken into very bite size chapters and has lots of reference pages.

 There's probably lots of other sites, but if you're just looking to get your feet wet and/or fill in a few gaps, give it a try and let me know how it worked for you...  --ToK