W3C Reference Note on Selectors and States: https://www.w3.org/TR/selectors-states/
It is part of the suite of specs that came through the W3C Web Annotation Working Group: https://www.w3.org/annotation/
More examples in W3C Note Embedding Web Annotations in HTML: https://www.w3.org/TR/annotation-html/
Different kinds of Web resources can combine multiple selectors and states. Here is a simple one using `TextQuoteSelector` handled by the https://dokie.li/ clientside application:
http://csarven.ca/dokieli-rww#selector(type=TextQuoteSelecto...
A screenshot/how-to: https://twitter.com/csarven/status/981924087843950595