In the help topic for text-indent
http://www.w3schools.com/cssref/pr_text_text-indent.asp
I think you should add a note that text-indent is commonly used with "em" since that creates indentations that are relative to the chosen font size instead of being a fixed number of pixels.
The "Play It" button for the "length" option should have a radio button for "1 em" and "2 em".
http://www.w3schools.com/cssref/playit.asp?filename=playcss_text-indent
I didn't know anything about "em" and had never seen it until someone else saw the difficulties I was having with trying to set fixed pixel sizes. Having that more on this tutorial page would have helped me.