Selection.anchorOffset
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2015年7月.
只读属性，返回选区的锚节点（ Selection.anchorNode）起点偏移量的数字。返回值从零开始计数，如果选区从锚节点（Selection.anchorNode）的第一个字符开始，返回值为 0。