What HTML elements are not tabbable even with tabindex?

Question 1

Since no one has yet come up with any kind of definitive list, I did some testing on a fairly recent version of Chrome and came up with the following list of elements which are not tabbable at all:

<base>
<basefont>
<embed>
<head>
<link>
<meta>
<object>
<param>
<source>
<style>
<title>
<track>

Unfortunately, since I only spent a couple of hours trying to write this list, it has a few caveats:

Not tested on any browser other than Chrome
I skipped over most of the elements that I thought likely to be tabbable (as they would have normal displayed content)
I have not tried setting any of these to be displayed (apart from <title> as in my comments)

I was most surprised that the following elements are tabbable:

<audio> and <video> (surprised because <embed> and <object> are not)
<br> and <wbr>
<col> and <colgroup>
<frame> and <frameset> (although they are more of a special case as they are not technically valid elements in HTML anyway)
<html> (there is almost no difference between this and <body> from a tabbing perspective)
<optgroup> and <option> (although tabbing to them does not give any visual indication - i.e. the select box does not open)

Question 2

The HTML spec lists conditions under which elements are focusable and how tabindex is interpreted.

The element's tabindex focus flag is set.
The element is either being rendered or is a descendant of a canvas element that represents embedded content.
Neither the element nor any of its ancestors are inert.
The element is not disabled.

The definition relies on elements being rendered, and with CSS you can make any element rendered. For example, a focusable <param> and even <title> and <basefont>:

<!DOCTYPE>
<title tabindex=0>Test</title>
<basefont tabindex=0>
<style>
head, title {display:block}
basefont, param {content: url(image.png);}
</style>
<object><param tabindex=0></object>

BTW: Ignore W3Schools — generally that's not a reliable/authoritative source.