This appendix is a very high-level glance at the general principles of SVG and its implementation in Inkscape. Some of this material has already been referred to in other chapters. However, this book does not intend to replace the official SVG specification. If you want an absolutely final, complete, and authoritative reference on SVG, you need the SVG specification produced by the W3C. Available versions of this specification can be found at http://w3.org/Graphics/SVG; the version that Inkscape currently supports, 1.1, is at http://w3.org/TR/SVG11.
Knowledge of SVG and, more generally, XML (SVG is just a specialized variety of XML) is not strictly necessary for mastering Inkscape—but it really helps. It will allow you to peek under the hood of Inkscape drawings, and it will often reveal the true reasons behind some of Inkscape’s features (or lack thereof). As XML is so widely used these days, this appendix’s material may also be useful to you in a lot of other situations not involving Inkscape and SVG at all.