5.4. The ‘symbol’ element| svgwg.org
Chapter 15: Scripting and Interactivity| svgwg.org
The Document interface represents any web page loaded in the browser and serves as an entry point into the web page's content, which is the DOM tree.| MDN Web Docs
Introduction| w3c.github.io
1. About the CSS 2.2 Specification| drafts.csswg.org
Status of this Document| www.w3.org