UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0

Example URL:
https://jsfiddle.net/tyjx5m0e/7/

Steps to reproduce the problem:
1. Create a content editable div
2. add some text
3. add a span tag inside the the div
4. try to step to 0 position with the caret in span tag

What is the expected behavior?
It should be able to set the caret position to 0 inside span tag.

What went wrong?
instead of caret placed at the front of the span, inside it, the caret paced after the first letter.

Does it occur on multiple sites: Yes

Is it a problem with a plugin? No 

Did this work before? No 

Does this work in other browsers? Yes 

Chrome version: <Copy from: 'about:version'>  Channel: n/a
OS Version: 10.0
Flash Version: Shockwave Flash 22.0 r0

When we want to go with the caret to the next node, we are unable to set the caret position to 0. So if we go inside, or leave the span tag, the caret step one more as expected.