UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2843.0 Safari/537.36

Example URL:

Steps to reproduce the problem:
HTML Standard says that default value is 0 for elements that are focusable and −1.
https://html.spec.whatwg.org/multipage/interaction.html#dom-tabindex

Some non-focusable elements (listed below) return 0 for tabIndex.

a without href
input[type=hidden]
input[disabled]
select[disabled]
textarea[disabled]
button[disabled]
filedset
output

What is the expected behavior?
Non-focusable elements should return -1 for tabIndex

What went wrong?
Non-focusable elements return 0

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: 55.0.2843.0  Channel: canary
OS Version: 10.0
Flash Version: Shockwave Flash 22.0 r0

It may be too late to change default valule of following elements as Firefox, Edge and Safari return 0 for years.

a without href
input[type=hidden]
input[disabled]
select[disabled]
textarea[disabled]
button[disabled]

Firefox returns -1 for fieldset and output