Svg Path D Generator. The path data from adobe illustrator does not. It is probably also the hardest element to master.
There are four other commands that are essentially simpler versions of the line commands. Semicircles that complete the circle in one path. Is there a site, say something like jsfiddle, on which you can paste in an svg path, get it valid.
In svg 2 and the latest versions of chromium browsers, gettotallength() can be used on any shape element, not only a <path>.