Point

A serializable representation of a boundary point in a DOM Range.

The cssSelector field always references a DOM element. If the original DOM Range start/endContainer property references a DOM text node, the textNodeIndex field is used to complement the CSS Selector; thereby providing a pointer to a child DOM text node; and charOffset is used to tell a position within the character data of that DOM text node (just as the DOM Range start/endOffset does). If the original DOM Range start/endContainer property references a DOM Element, then the textNodeIndex field is used to designate the child Text node (just as the DOM Range start/endOffset does), and the optional charOffset field is not used (as there is no explicit position within the character data of the text node).

https://github.com/readium/architecture/blob/master/models/locators/extensions/html.md#the-start-and-end-object