Improve docs on link format

Author: CrazyPython

README.md

@@ -13,5 +13,4 @@ The redness represents how active an API's proposal or specification is.
 * Date links to the Specification
 * Element abbreviation links to What can this do? (Demo, Playground, or Reference)
 * Details links to How can I learn this? Guide
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+* Background links to browser support (For most APIs, caniuse.com. For in-progress APIs, implementation progress pages.)